Is there a way to check and see how many of a custom item are on the site?

Nope, sorry ):

There totally should be though. I would like to know about some, and I can't remember how many batches I released of the song of dragonkin. dies

I think you can check on subetalodge? When I searched for 'custom items' It says there is 7034...

Edit - ah, you mean how many of a certain item - No, there isn't a way to check that I don't think D: