1690 pets
Currently there is a cap of 500 companions in the Pet Journal on the beta. With a possible limit on the max number of pets a player can have in MoP, duplicate or unwanted companions may seem inconvenient.
The good news is that there will be a new ability in MoP to "release" pets - essentially removing a companion from your Pet Journal without having to put it into a cage. Using the release ability will help make juggling and organizing a collection consisting of hundreds of pets less troublesome!
However, it was initially possible to release any and all pets (even the very rare and hard to obtain ones), and this proved to be problematic since there was no way to retrieve said companion if it was released on accident.
According to a recent blue post, thankfully a measure has been taken to help safeguard collections and those extra rare pets from accidental deletion. [source]
We have added restrictions on releasing pets in the latest build. The only pets you can release are ones that you have caught though the pet battle system.
Fun-filled Facts
The Pet Journal is shaping up to be a very useful tool for Pet Battles as well as collecting, and it was updated with pet descriptions! Many reveal interesting and often amusing tid-bits about each pet.
For example: "Shy Bandicoon - A goblin alchemist once claimed that the bite of this raccoon gave him the gift of prophecy. Turns out, it was just rabies."
Despite many companions sharing similar models, each pet has its own unique flavor text under the Info icon in the journal preview window.
You can check out the WarcraftPets forum thread that lists many of the pet descriptions. MMO-Champion also has a transcribed list of the companion flavor text.
There are quite a few to read through, but these pet descriptions are sure to brighten up anyone's day!
