I'm interested in running this game, but I have a quick question. Is co-op only 2 players, all 4 players, or any amount more than 1?

Any amount more than 1 should be fine imo. Having less then four loses a lot of time due to the AI players frequently getting stuck on the terrain.

I actually think that three players is better than four. When you have less than four players you can cancel many attack animations by quickly switching between characters. This saves a lot of time throughout the run. While four players would be advantageous during stretches of the run where attack cancels are not needed, I think that three players will be faster overall.

Of course, if you could have a fourth player who plugs in and unplugs his controller multiple times throughout the run, that would be even better.

The character swap animation cancel is a neat bit of tech I didn't know about :D Having four players for when that's not necessary would be really good though, we could potentially route in getting enough coins to buy the attack upgrade in the spooky forest which saves a lot of time throughout the run. It might even be worth farming bonus2 a few times before doing walking the path to buy yet another attack upgrade. I'm not sure how much time that would save later on though.

If you want to see the animation cancel in action, watch the WR any% run. It has several other tricks/glitches I have not seen anyone else do yet as well.

Regarding the Spooky Forest attack upgrade, it saves about 2:30 (EDIT: it looks like it only saves 1:57) on Ogre Killer and probably a few seconds elsewhere, so whatever you do to buy the upgrade needs to take less than 1:57. There is just a little bit over 400G prior to the shop in the first two levels. I do think it is worth looking into in a multiplayer run.

I have recently been considering going back to Spooky Forest immediately after completing it to buy the upgrade with the money collected during Donkey's Hero Time. It takes a little under 2:30 to get to the shop in my run, so it is probably a little bit too slow to save any time.

The second attack upgrade costs 1400G, which will almost definitely take too much time to gather to be worthwhile.