I can start bitcoin core loading with 200 wallets. But when I increase it to 300 wallets, it fails. Below is the debug.log when I tried to load 300 wallets.
2018-12-17T01:36:27Z init message: Loading wallet...
2018-12-17T01:36:27Z [wallet-201.dat] nFileVersion = 170001
2018-12-17T01:36:27Z [wallet-201.dat] Keys: 3 plaintext, 0 encrypted, 3 w/ metadata, 3 total. Unknown wallet records: 1
2018-12-17T01:36:27Z [wallet-201.dat] Wallet completed loading in 1ms
2018-12-17T01:36:27Z [wallet-201.dat] setKeyPool.size() = 2
2018-12-17T01:36:27Z [wallet-201.dat] mapWallet.size() = 0
2018-12-17T01:36:27Z [wallet-201.dat] mapAddressBook.size() = 0
2018-12-17T01:36:27Z init message: Loading wallet...
2018-12-17T01:36:27Z [wallet-202.dat] nFileVersion = 170001
2018-12-17T01:36:27Z [wallet-202.dat] Keys: 3 plaintext, 0 encrypted, 3 w/ metadata, 3 total. Unknown wallet records: 1
2018-12-17T01:36:39Z [wallet-202.dat] Wallet completed loading in 12012ms
2018-12-17T01:36:39Z init message: Rescanning...
2018-12-17T01:36:39Z [wallet-202.dat] Rescanning last 1447907 blocks (from block 0)...
2018-12-17T01:36:39Z [wallet-202.dat] Rescan started from block 1447885...
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=14930383)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=14955195)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15731604)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15152698)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=14994412)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15000180)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15237899)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15073409)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15076691)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15162221)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15175705)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15238839)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15734430)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15746232)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15755504)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15759768)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15780715)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15795130)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15814178)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15841983)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15844548)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15870120)
2018-12-17T01:36:39Z Unable to open file /root/.bitcoin/testnet3/blocks/blk00155.dat
2018-12-17T01:36:39Z ERROR: ReadBlockFromDisk: OpenBlockFile failed for CBlockDiskPos(nFile=155, nPos=15897975)
2018-12-17T01:36:39Z [wallet-202.dat] Rescan completed in 1ms
2018-12-17T01:36:39Z [wallet-202.dat] setKeyPool.size() = 2
2018-12-17T01:36:39Z [wallet-202.dat] mapWallet.size() = 0
2018-12-17T01:36:39Z [wallet-202.dat] mapAddressBook.size() = 0
2018-12-17T01:36:39Z init message: Loading wallet...
2018-12-17T01:36:51Z [wallet-203.dat] Releasing wallet
2018-12-17T01:36:51Z BerkeleyEnvironment::Flush: Flush(false)
2018-12-17T01:36:51Z BerkeleyEnvironment::Flush: Flush(false) took 0ms
2018-12-17T01:36:51Z
************************
EXCEPTION: St13runtime_error
BerkeleyBatch: Error 24, can't open database wallet.dat
bitcoin in AppInit()