The maximum should be 1000 but this can depend on your system. You would probably need a ton of RAM to open that many.
I have read that "1000" is default maximum for Acrobat 4 or Acrobat 5 (I don't remmember it exactly anymore). Concerning my PC and RAM. I have a high-end graphical computer at my office; so I'm sure the RAM memory is not the reason.
Anyway, thanks for answer.