[davidw]

If you can run 3.6.4, you shouldn't have any issues running 3.6.8 (It requires the same version of PHP & MySQL). However, future versions may require change on that note.

As for template changes, to answer your question...
Yes, there will be template reverts, but to "redo" them, you won't necessarily have to. Some templates will be automatic, others will be small modifications depending on what hacks you have installed or custom customizations. One thing you can do is compare your code with the new version. Keep in mind no templates will be made until you change them or revert them. You will (upon completion of the upgrade) see a list of templates that need updating:

Out-of-Date Custom Templates Found!
There are currently x customized template(s) that need to be updated or reverted. Some sections of vBulletin may not function if you do not do this!

Click here to view these templates!

Once there, you will see those templates sorted by style

Style Name
FORUMHOME	                             [Edit Template]
Default Template ...                          [View History]
Your Custom Template Last Edited ...                [Revert]

You can click on View History to compare the two versions (your version and the new version) and do a side-by-side comparison to see the changes together.

As for how many, that depends on what hacks you have installed. No, you will not have to work on all of them, but it may very well be at least one or more than one template.