But what happens when the number goes down?

Danghiu!

Why wouldn’t user just send bitcoin to anyone user via lightning

I'm lost at this ... So I thought that mint should keep bitcoin by LN receipt and then it gives back ecash to user (like a kind of bank). Here in the draw I saw that user pay directly the LN receipt to the exchange but mint will release anyway ecash to the user. So I mean that it seems from the draw that mint is giving ecash to the user but it doesn't get sats on change, that are paid directly to the exchange. Probably I'm missing something