Edit /etc/system and add the following line:
* System V pseudo terminals set pt_cnt = <num>
Halt the system and boot -r.
You can essentially have as many as you like, but you'll probably run into some other limit somewhere. More than 3000 are supported.
In the unlikely event that you run out of BSD-style ptys, you can increase them as well. The maximum here is currently 176 for pty[p-z][0-9a-f]. This is somewhat less that the BSD maximum of 256 limited by 8 bit device minor numbers.
BSD ttys are awkward to use and all programs I found support SYSV ptys without trouble.
* You don't need this. Increasing this value too much usually * just wastes memory. * BSD application never support more than 256 ptys. * Solaris 2.x supports no more that 176 BSD ptys. set npty = <num>
But you're not there yet, you also need to edit /etc/iu.ap and substitute the new value of "npty-1" for the "47" on the following line, in case you do increase the number of BSD style ptys.
ptsl 0 47 ldterm ttcompat
Halt the system and boot -r.