How many total transactions are there in each Ardor block? Out of the total transactions per block, how many are child chain transactions? What are the limits and what types of scaling solutions are being developed to address these limitations?
If I'm right, the current hard-coded limitations are:
100 tx/block for main chain (block spacing is 1 minute)
100 tx/block for every child chain.
The current maximum capacity (if there were 100 child chains) would then be 10000 tx/minute (or about 160-170 tx per second), because child chains' transactions are bundled into a main chain ChildChainBlock transaction. A more realistic value would be 5000 (with 50 child chains), so there is space for 50 ARDR transactions per minute.
I guess that's largely a anti-spam measure and can be lifted once it becomes necessary. But even the "realistic" 5000 tx/min would mean about ten times Bitcoin's capacity. It would need relatively powerful nodes, but once child chain pruning is implemented, it would become much faster to sync and "catch up".
Waiting for an official developers' answer though ;)