In the very early incarnation of the patch 5.3 PTR, hunter stables have been expanded to accomodate up to 50 hunter pets. This awesome feature was discovered by a score of hunters almost as soon as the PTR was available, though I believe Kalliope reported on it first.
With this change hunters will now be able to collect up to 55 different pets, with 50 in the stable and 5 in the active pet slots.
In the comments to this news at the WHU, Arth made an apt observation:
What indeed, hunters? Let's take a moment to look at why hunters need so ridiculously many stable slots, and why we got them.
Min. range is gone, our specs are fairly well-tuned and are all viable, we aren't terrible in PvP, and now this. What the hell are we supposed to complain about?