Let's see. . .

yea no...

bruh
i don't blame you

lol

I can't name all of them, but I can name as many as I can
Last breath, dusttrust, something new, lethal deal, murder time trio/mad time trio, underswap, storyshift, deltraveler (if that counts as an undertale AU and not a deltarune AU), all together, underverse, xtale, glitchtale, call of the void, help from the void, disbelief, dustbelief,

Undertale red, undertale red and yellow, underfell, fellswap, swapfell, dusttale, I would say reboottale counts, reapertale, inktale, dreemurr reborn, no more deals, great time trio, bad time trio, handplates, underplayer, aftertale, whatever AU the omega timeline is from, whatever AU VHS sans is from, horrortale, undertale don't forget,

Ans that's all I think I can name

*and

Yo ''Reboottale'' (my AU) mentioned :D

Correct

Here's all the ones I've heard of (I haven't seen most of them)
Dusttale, glitchtale, axetale (which i was told started good except for the frans), Inverted Fate, swapfell, fellswap (what even is the difference), underswap, underfell (i think its called that), inktale, disbelief, dustbelief, epictale, helptale (if my sources are correct, peak horror), UT yellow, horrortale, last breath, something new.