How many rejections on fulls would one accept before they try to "fix" their manuscript?

I have a group of very honest, very sharp beta readers who help me beat my MSs into shape before I query them. So by the time I query the MS, I'm pretty confident about it. It would take at least five form Rs on the full before I start to question the MS. And even then, I'd be hard pressed to know where to start fixing the problem, because reasons for Rs range so wildly. (The last time I queried, one agent rejected because the MS was "too commercial", and another agent rejected because the MS was "too literary, not commercial enough". Soooo yeah.)

Hopefully, the Rs you get will be personalized.

Well, hopefully, you won't get Rs, just offers.

Rejections are simply part of the game and you can't jump to conclusions on only a few rejections. You'll stack up a few. The big thing is whether you're receiving any feedback. If the rejections stack up and you're getting no feedback, consider that something may be wrong with the MS.
Typically, if an agent invests more than a 50 pages of reading, they'll mention something (not always the case, of course, but quite often it is).

It's like picking out a car - you'll look at dozens that you glaze over and you won't talk about those because they didn't catch your interest. But the ones you take time to evaluate closely, you'll talk about, ask about, and inquire on. It's an investment, after all.

For me, it would depend. Most of the full requests, I'm assuming, would be off a partial, so I'd want to look at how the rest of the book stacked up against the initial excerpt that had interested the agent. If I really couldn't get any traction at all, I'd probably consider starting over with a new book before I'd attempt a complete rewrite, but it really would depend.

The agents who've read the full requests I've sent usually did not read all the way. A lot of times it's hugely subjective. If you're ready to query, I would make sure you have the manuscript as polished as you can make it, and HAVE CONFIDENCE in it If you know that it's the best you can possibly bring it to, then don't change just based on one agent's feedback (unless their feedback really speaks to you).

I typically never revise because of form rejections. If I am getting personal rejections that all point to the same issue, then I do start to look at revisions.

While you're waiting, begin writing the next book.

Once you've started to query--and especially once fulls are being sent--that MS is more or less done. Doesn't mean you can't re-open it, but it should be the best of your current ability already. If you don't have feedback, you don't have much to revise on--save the time for an R&R, if you get one. Remember to be persistent and patient.

Then, if MS #1 is still going nowhere after awhile, you have MS #2 ready, to the best of your ability--except it's likely that just the process of writing and polishing the new book will make it better, more polished, more professional.

Or, it might take until MS #4, or #5, or so on. Persistence and patience, right?

Or, you might get a late bite on MS #1 when you least expect it. Or on MS #3 which then helps sell MS #2.

Don't give up. Don't fall into the trap of only ever writing one book/story. You can always come back to that first, but that might not always be the best way to move forward.

I'd +1 what Putputt said, in that beta readers can help. Even then, though, there's no substitute for just writing. You will always get better.

It's hard to know where you stand without any feedback other than a vague *something* is wrong. However if you get consistent feedback from agents saying the opening is too slow, the voice is off, can't connect with the characters, etc...that would be something to think about revising. But I personally wouldn't do a significant rewrite based on a form rejection, or for that matter even more personalized feedback from just one agent (unless you tend to agree wholly with what they are suggesting).

Just my 2 cents.

Totally agree with VeryBigBeard on this one. Sometimes when the book doesn't land an agent, it's best to move on to other works. You'll get better and gain skills with every new project, not necessarily trying to hit the mark by re-drafting multiple times. That being said, in my past two times querying I've had at least one agent give me useful feedback. As others said, it's still subjective, but for my last project I had a couple agents cite the plot as their main reason for rejecting. The first half worked, the second half lagged. I came to the conclusion that the book concept itself just didn't quite work and I moved on to something else. It's probably a project I'll return to in future (in a revamped form) but at the time I knew I didn't have the skill to truly make it cohesive. But it was useful to learn from that feedback so I knew before starting the next project.