Hi Keith,
There is no set maximum limit.
The total of EICLASS MPL defines how many event/job related requests can be processed concurrently. And if there are more requests than that, they will be queued in ESP CKPT file.
So the size of the CKPT can be a physical limit on how many requests can be queued.
I suggest that you can set up an ESP batch to issue OPER LISTCKPT when ESP is busy, and see if it's approaching its maximum size.
Hope this helps,
Lucy