I've heard from a few students who are under the impression that you're allowed only two extensions on homework assignments for the entire semester. That's a misreading of the information on the WebAssign page. What it actually says is, "You can obtain a maximum of two extensions in this way on each assignment." So there's no reason to hoard your extensions.
The reason for not relying too heavily on extensions is that you can only get 75% credit for work done using an extension. So you want to minimize use of them for that reason. But you don't have to worry about exhausting your supply of extensions, because extensions used on one assignment don't in any way decrease your supply of future extensions on other assignment.