Is there a limit on the number of stocks I can short?

Margin details can be found on the Margin page of the Interactive Brokers website, which you can access by clicking the Products menu option followed by…

A Short Sell order will be accepted and can possibly trade if you have a margin type account and there is sufficient Available Funds to meet any change in the account's margin requirements. If If you do not want the ability to sell short, you would need to have a Cash type account as Cash accounts cannot borrow shares of stock to naked short sell, be short naked Call options or short non-cash secured Put options. Cash accounts are restricted from holding a short balance in any currency as this would also constitute a loan from IBKR…

There is no feature that allows one to set a specific "Risk" limit for an IBKR account but clients do have tools available to help manage the risk of open positions.

The Account Window displays real time account values which allows one to monitor the account's margin compliance…

There is no limit on the number of FHSA accounts you may have. Contribution limit is aggregated across all your FHSA accounts…