when I buy a magic catcher the number of them doesn't go down... I didn't keep track of it because I just now noticed it...

Magic catchers are a premium item and can be purchased only for gold coins. Please obtain some gold coins if you want to purchase them.

that's not it I bought them and it still said I had twenty coins and this happened a few times. Didn't manage to catch anything so I wound up buying stable space with the coins and now I have the anticipated zero...

Ok, we dug the logs a bit. It looks like everything works as expected. Unlike regular catchers who stay with you until they meet a pet which is too strong for them, magic catchers can be used to catch only one pet. You used your Magic Catcher to catch this unicorn, and then he left. Buying a magic catcher and sending him to an expedition results in the same number of available magic catchers that you had before the purchase. You will have him available for the next expedition only if he returns empty-handed.