Well joining in on the fun of this speculating prediction game...

I'm actually going to say zero!

I say that because:

A) By that time they will have probably landed several high altitude hop tests successfully.

B) I don't think the heat-tiles will give them as much problem as people are anticipating, because mainly:

If the tiles survive a lot of prior hop tests and seem pretty securely attached, and the tiles survive a lot of heat blasting tests with blow torches!... Then it's not too much of a stretch to see them surviving the first few orbital re-entries just fine.

For example: even given all their issues and design problems, the shuttle tiles survived the first many re-entries just fine, and nailed the first re-entry test with STS-1 in 1981.