Right, technically a human only has 1 hole for ingesting nutrients and expelling waste as well though, assuming you are talking about the mouth and anus. Does a flatworm have any other through holes though e.g. nostrils or tear ducts? I have no idea what the biology of a flatworm is haha.

By that logic a tire has 0 holes in it by default, so a straw should have 0 holes as well :)

IIRC humans have 7 holes topologically (assuming both vsauce and my memory are correct). I'm not sure how many a flatworm would have, but I bet you could group animals by number of holes topologically, which might be interesting.

There are only two real answers, either 0 or 1.

If you are going topologically the answer is one, just like a donut.

Personally I like to think of a straw as a hollow cylinder of plastic, in which case the "hole" in the center is just space the cylinder doesn't occupy and not actually a hole, so zero.

Two holes doesn't even make any sense imo.