Not many really... There were some rare cases. But the bulk of production had no rear. For most purposes, including the majority of competitions available, you would want to try it the way most of the troops at Waterloo did.
The Military muskets were designed for a purpose and designed well. Of course, most were designed better then the Brown Bess, but that's a different story.
Effective range of Charleville up through the 1842 Springfield is 100 yard plus. With stories of original shooting suggesting a certain amount of expectation at 200 you end up just trying to find out what can be done.
It seems a shame to give up on "old" skills and opt for "Smooth rifles" just because there is a perceived advantage to having a rear right. While the NMLRA has matches for both, which means having a good excuse to buy/build two guns instead of one, MLAIC competition is military style only.
Frankly, having a rear sight on a smoothbore just tends to specialize the gun for a specific match-distance...and cuts you out of a lot of matches. Without the rear sight you can hunt or shoot any match, any range, and with a little homework, just as well if not better.
Once you give in and go for a rear sight you're on the slippery slope of technological decline that takes the human factor out of shooting...and a lot of the fun too.