If you over-use static port, pf won't allow one to "steal" the other's connection, but will deny the new connection (can't create a conflicting state).
It would only be denied iff it tries to re-use the same static source port going to the same destination:
Connection #1:
a.a.a.a:xxxx NAT to b.b.b.b:xxxx destination c.c.c.c:yyyy
Connection #2 – Denied:
a.a.a.q:xxxx NAT to b.b.b.b:xxxx destination c.c.c.c:yyyy
Connection #3 -- OK:
a.a.a.q:xxxx NAT to b.b.b.b:xxxx destination d.d.d.d:zzzz
Limiting use of static port to things that require static port (e.g. a PBX) is the best way to avoid problems. Generally speaking, the only time someone would really hit scenario #3 above is if you have two local PBX units attempting to connect to the same remote PBX. In that case you'd need to NAT one of them out a different IP address (1:1 NAT or outbound NAT to a different IP address)