So, generally speaking, you get to attack once for every natural attack you have. Gain another natural weapon, gain another natural attack.
However, natural attacks are a muddled confusing mess, in both D&D 3.5 and Pathfinder. Most of the rules overlap (and many of those are confusing), but some of the rules differ (and those are usually just two different flavors of confusing). This question is one of the big ones in this regard.
The consensus, in both D&D 3.5 and Pathfinder, is that gaining a natural attack does not automatically mean you have the limb you need to take it. So if you gain two bite attacks, but only one mouth, you have to pick one to use in any given full-attack. You could pick the other one the next time you attack, if you wanted, but you can’t use both during the same full-attack. Likewise for claws (can’t use more claw attacks than you have hands), wing buffets (can’t use more of those than you have wings), and so on. Gore attacks are usually one per head, even, rather than one per horn or whatever. The one usual exception is tentacle attacks; those usually come with the tentacle you need to make the tentacle attack.
Where is this stated in the rules? I could have sworn it was in there, but I’m having an awfully hard time finding it. Like I said, the rules in this area are a muddled mess. But it has been suggested by developer commentary, and is kind of implicit in the way natural attacks are handed out (e.g. the girallon arms soulmeld has to explicitly give you an extra pair of ghostly arms so you can actually use all four of the claw attacks it offers). It’s also consistent with the rules for mixing manufactured and natural attacks, which are explicit about how a hand that attacks with a sword has been “used” and cannot also attack with a claw.
This is certainly how I have always played and always seen things played. Even with this limitation, it is entirely possible to get unreasonable numbers of natural attacks even as it is (try looking up one of the various King of Smack and derivative theoretical-optimization builds for this taken to truly ludicrous extremes). Without this limitation, natural attacks would very quickly and very easily get out of control.