If you split your attacks between targets, hitting each twice, it seems clear at least in that case that you would get rend once on each target.
However, for the purposes of more rending on the same target by hitting that target more and more, the wording is really unclear and you’ll have to have to ask your DM.
The contention is this:
If you hit a single target with at least two claw attacks...
The at least wording implies that having more than two hits doesn’t trigger new rends... but it doesn’t really outright say so. Moreover, there’s no timing information given: when does that reset? I would guess that the intent is something like “if you hit a single target with at least two claw attacks during a single action,” or something similar, but it doesn’t say that. It could alternatively be a per-turn limit or per-round limit, or it could be something that can only happen in a full-attack (and then only once?). Taken literally, it seems like it would just be as soon as you got a second claw hit ever against a given target, you would get your first and only rend on that target ever, which seems wrong.
Anyway, I would ask your DM. My inclination is that it’s limited to once per action, and the two hits have to be scored during that one action (so a full-attack, or that feat that lets you attack with two weapons in an attack of opportunity, or whatever). Getting more than two claw attacks, e.g. with (Improved) Rapidstrike, doesn’t seem likely to help.
On the other hand, getting rend more than once hardly seems overpowered. Rend is a rare ability and it’s almost never available in large amounts. It rewards you for doing what you want to do anyway, so that could be appropriate as long as you don’t have to pay much to get it in the first place, but ultimately it’s pretty minor; not something you can really build a character around. Just a little cherry on top. If you want it to be more than that for your character, you might ask your DM to let you rend more (every two hits, every hit after the first, whatever) and argue that what you’re investing in getting rend is worth that much bonus damage. In my experience, this should hardly be game-breaking.