Holy !$#@ !!! Why do you have so many instances ? Are these test or production environments ? If this is production, how many concurrent users are using the current instances ? How big are the instances ? Remember, each instance cuts your resource pie significantly and that informix does not handle these separate shared memory segments efficiently.
The physical limit is 255.
I would advise to set them all to tcp connections, because shm could cause problems.
'onstat -' shows you the amount of memory allocated.
in yr $ONCONFIG, the parameters BUFFERS, SHMVIRTSIZE and SHMADD affect most of you mem.