Let's say I have an imported-type electrum wallet, how many private keys can I import to that wallet, if there's a limit to it?
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Theoretically, no. A wallet is simply a collection of private keys (or a root key + scheme to derive more private keys, such as with BIP39 wallets).
However, after a certain point, you will start facing issues of scale. Most wallet applications will struggle to index transactions for hundreds of thousand or millions of addresses. Exchanges and other companies that deal with that many addresses will often build out special software tooling to handle tx indexing, coin selection, security, and other use cases that go up in complexity as the number of keys increase.
But from a purely crypto point of view, nothing prevents you from having any number of addresses in a wallet.
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1I'd like to add that in the case of electrum you will likely face problems with even a few thousand addresses. That's because it relies on electrum servers for transaction data and those are run by volunteers so they place limits on how much data you can query. A full node client like bitcoin core will scale better because it has the blockchain locally. – Abdussamad Nov 18 '18 at 11:59
