Once I hit 165 the game crashes on start up.

From reading, I think the actual limit may be 255. But there could be plenty of other reasons why you're crashing at 165. A mod that finally is one too many conflicts/invalidations, or finally too much extra load for your PC to handle, etc.

Does it matter which mod is the 165th (in terms of crashing?) or is it only when you're trying to load a single specific mod that just happens to be the 165th?

It doesn't seem to matter. for testing I just duplicated one a few times. I was spending my time turning them all on one at a time to find what was causing the crash and noticed that now every one I enabled caused the crash. I think I will have to create a merged patch of some of the smaller ones.