Torulf, whether or not there's a limit depends on if you're using Poser 4 or 5. On Poser 4 there is a limit of 256 entries in each folder, but long before that limit is reached, you will notice that the names becomes truncated, up to a point where only a few letters are visible. If you exceed the limit you just won't be able to access the stuff at the bottom of your list. In that case, your only hope (short of deleting) is to get PBooost. (from the MP here) But as far as I know Poser 5 hasn't got that limit.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.