There's no limit at all, other the hard drive space to hold all your .INI file. Maybe there's a problem with one of them, or a problem with the AFCAD you are using.
You can also activate a log, to see what's happening: open the Notepad and type the following on a new blank file:
logFile=couatl.log
(please note logFile is written beginning with a lower case l and a capital F )
Save the file as couatl.ini in the FSX\fsdreamteam\couatl folder, so it stays at the same level of the couatl.exe. Then go in FSX, then when it's done loading the scenery you can't edit and ready to fly, then Exit from FSX and post the content of the Couatl.LOG file that should have been created inside FSX\fsdreamteam\couatl