The number of assets available for borrowing is determined proportionally by free pledge — the amount of unallocated Collateral Balance funds not used to support positions or loans.
A collateral equivalent is required for any amount of the loan asset and depends on account leverage. Account leverage determines a maintenance margin fraction ratio between 50% (1x leverage) and 2.5% (10x leverage). Increasing account leverage would allow you to borrow more funds, up to the almost equal amount on the market. However, it is connected with additional risks, such as position liquidation. In that case, it is really important to check the account risk score and add more funds to the collateral balance to avoid liquidation.
Open margin and futures positions impact the amount of free pledges: the larger the positive value of PnL (Profit & Loss), the more is available for borrowing.