More than 2 can be in a cross zone pool, however various programming options would determine how they'd behave.
If you have self verification enabled, then that would mean the same detector generating an alarm event twice within the time specified by the panel will generate an alarm at the panel.
If this is not enabled, then if any two of the hardware zones you put within the pool trip, then you'll get an alarm at the panel.
The generics of it would be the first trip of a zone within the pool "arms" the event with the second trip of the same (verification enabled only) or different (pool) detector would set the system off. It makes no difference how many detectors are in the pool, since they're all common to each other.
Then the time selection would determine if you can have the first trip expire before another event happens from within the pool which would either generate an alarm or not generate an alarm based on the time value selected..
Think of it like 2 open switches wired in series, the first closes for 5 seconds, if the second closes within that 5 second period, you have a circuit. If it's longer than 5 seconds, no circuit. In the case of a pool, it's the equivalent of the panel seeing a pair of switches, no matter what the order, as long as the "zones" are contained within the software point of "cross zone pool".