When you get a lot of feedback at once...

Everything Maryn said.

I just wanted to add, give yourself time. For me, it takes a while to wrap my head around things, gnaw on solutions, etc. Take one thing at a time, and don't worry if the process feels messy. It can be, but you can keep moving through it.

Ah poop. Just had my first response eaten when I hit the wrong button. >.<
*sigh*

Ditto what Maryn said. Personally, I also find it useful to work in a copy of my manuscript and note chapter by chapter (or in a OneNote file, whatever works for you), the common high level things my betas all agreed on. Those are first priority, as they'll probably require the most revision, and it's safe to say if everyone's pinging me on it, it's an issue. How I address it is the tricksey part...

Then, I make note of the things that individuals pinged me on or where there was difference of opinion. I'll usually chew on those for a couple days and listen to my gut. I may also ping some of the other betas if they didn't make a comment on that issue, just to see what they think. Sometimes I'll get a "oh yeah, I think I noticed that too, but not enough to say something?" Or they will be like "I will CUT YOU if you change that" and then you can start to suss out if it's an individual reader preference issue. Good times.

It may be tempting to go after the sentence level stuff, but resist! You should address the high level structural issues because a lot of those sentence level things will go the way of the Dodo in that process. But do make a "GOSH DARN IT" list with your, um, lovely little idiosyncrasies, aka what I like to call my "BAD WINKS HABITS" list that I'll use when ready for the polishing round later.

Good luck! You've got this.

I read through feedback first as just a read-through to get a feel for it. As in, is this valuable overall? Then I'll let it sit for a few days and then go back and examine each point with my document open. If I feel it's something I need to address, I'll make a comment in my own document. (size of the issue is irrelevant at this point - it's as simple as "do I need to address this")

I automatically give more weight to things that multiple people mention. In my newest one, 3 of my betas immediately pointed out that I didn't have enough physical description for my characters, so that's something I'll address right away.

Eventually, I have all the feedback merged into my main document. Then I'll go in and make changes as needed.

Take every bit of feedback with a grain of salt, and don't weigh their opinions more heavily than your own. And TRY not to get discouraged. I know, getting a document back with a zillion comments about changes can be disheartening.