Constant MAX_OPEN_CEILING 

pub const MAX_OPEN_CEILING: usize = _; // 240usize

Most databases a pool will hold open, whatever it is configured with.

Derived rather than picked: max_open can come from a config file, and a file must not be able to talk a process into running itself out of file descriptors. Exceeding this would not fail at the pool — it would fail at some unrelated open elsewhere in the program, which is the worst place to find out.