You can use protocol data provider for `getReserveData` and compute the available liquidity (total supplied - total debt). This is the max amount that can be drawn from the Aave pool. Max amount a user can borrow depends on the collateral supplied. The limit is HF should not be <1 afterwards. You can use `getUserAccountData` method in pool to get user's total collateral and existing debt balance across all reserves https://docs.aave.com/developers/core-contracts/pool#getuserconfiguration and compute the max amount that can be drawn without dipping HF below 1. This [question](https://aave.peeranha.io/experts/1064/is-there-any-limit-amount-of-borrow-that-an-address-can-ask-for-on-aave-eth%3F) might also help.