I wanted to upload around 20-35 or so images from the same artist, but I don't know how much this would be accepted to upload so many. To try to not make it as spammish I'm planning to only upload a dozen or a day and wait until (or if) those get approved before uploading more on another day, but they are all more or less all of the same subject matter (Umbreon basically, a few also have the artist's OC in them). To clarify these are not a YCH or any other sort of variation of the same image, they are all different drawings of the character in different poses, but of the same character by the same artist.

Would this generally be accepted or be considered too spam-like?

yes, that's perfectly fine, you could upload them all at once if you wanted to.

As long as they don't violate anything in the guidelines and they're tagged properly, then yeah.

There's a hard limit on 30 posts every hour, though.

You won't have any problems. Bursts of image uploads like that happen a lot and, against the roughly 2000 uploads we get each day, it's not really a problem. Besides, as WeRideAtDawn points out, there are upload controls built into the system, so spamming won't be a problem for Users who upload properly and in good faith.

Probably should go without saying but make sure all of your posts are of the highest quality possible.
You don't want to end up in a situation where you have uploaded dozens of posts, only for them to all get deleted because there's a bigger version elsewhere.