Five. That is what I run and it is what I tell everyone in the group to run.
The reason is simple. You are most likely going to turn off one or two of those five because they are not performing. Starting with five means you still have three or four live ads after you prune, which is enough to keep scaling. Start with two or three and one bad ad leaves you too thin to push spend.
When you scale, more active ads gives facebook more to work with. If I am going to scale hard, I want facebook choosing from four live ads, not two. More options, more room to find the next winner.
Keep it clean inside the adset. One headline, one piece of copy, one creative per ad. Do not stack three versions into a single ad and hope facebook sorts it out. Let each ad be its own clean test.