We have hit the llimit of the  default value of  MAXUSERS (500).
The value can be increased dynamically  by issuing MAXUSERS xxxx, but after cycling the region it reverts to the default value. 
The new value needs to be retained across restarts and/or IPL.

Release: SLACCS00200-5-SOLVE:Access-Session Management
Component:

The parameter 'MAXUSER ####' must be added to to one of the start-up parameter files. 

I would suggest to add it to the 'User Initialization Procedure' – which by default is member $ACINIT found in your TESTEXEC library. 

The syntax for the statement to be added in the $ACINIT member is as follows: 
MAXUSER 1000 
 Note that this is not a SYSPARM; it is only the keyword followed by the value (number of concurrent users allowed).

After a recycle of the region, the N11302 message now reflects the new value  - 
N11302 MAXUSERS LIMIT NOW SET TO 1000. 

Also, when you display the option using the MAXUSER command, the message N11301 will reflect the new value:
maxuser
N11301 CURRENT MAXUSERS LIMIT IS 1000, NUMBER OF USERS IS 1. 
** END OF DELIVERED MESSAGES ** 

Additional information can be found in the Administering Guide.