How many witches could there possibly be? [She doesn't know how loaded that question is, what it means to Shiro, but...

She wants to know, even though it's not her place to know.]

So there's a downside to all of this magic? Why use the magic at all, then?

[There are other ways to fight. Other things that can be done that have less downsides than whatever the cost of magic may be.]

It seems...foolish. More danger than it would really be worth.

[She's not yet at the point where she'll push, and push, and push - until she can push no more and everything is falling apart around her.]