You can say "How much toast would you like?" or You could say "How many pieces of toast would you like?" In English we would never say "toasts" unless you were using the word as a verb.

Good instincts noting that "much" signals a non-count noun in this case! "Toast," like "bread" is generally non-count. The only thing that sounds ok to me is if maybe there is some specials dish called a "toast" and someone needs "five toasts." But that would be at the least atypical.

Yes, toast is a classic example of an uncountable noun. We never say, "Would you like some toasts?", but, "Would you like some toast?" is correct. Then my mother would have asked, "How many slices would you like?" (never "pieces" - but that is probably the difference between American and British English)