If anyone honestly think the GOP would impeach Trump, you're either naive or insane.
We don't need the whole GOP. Just enough to hit 218 in the House and 67 in the Senate. A tall order, but midterms + a hypothetical indictment make it much more attainable.
Would 15 GOP senators vote to impeach?
Well, if it gets to the Senate, it would be a vote to convict, so I assume it would depend on what evidence Mueller puts forward, which will presumably not be for a while. If you're looking for a list of 15 Republicans who might vote against Trump, here's roughly my list based on Clinton-state Republicans, those with a moderate/maverick reputation, and those with a personal beef with Trump:
1. Sasse
2. Paul
3. McCain
4. Heller
5. Flake
6. Collins
7. Murkowski
8. Portman
9. Gardner
10. Rubio
11. Lee
12. Hatch
13. Corker
14. Young
15. Cruz - it would certainly be interesting to watch the battle of "he said my dad killed JFK" vs. "I love how much everyone hates me" play out in Cruz's head
I would add Graham to this list as well.