In the unit overview (selectable from the top-left button), you can either double-click a unit - to select it and center your view on it - or you can click a unit and then, in the lower-left unit info box, either click its portrait or its name.
As far as I know there is no way to get an immediate overview of where all your units are, which is a shame - I would also be interested in something like that. They don't even appear in the mini-map in any form until after you select them...
You can find more information about the unit list (for example, what do the colors mean) in my answer to another question.
You get a -50% penalty if you are using a unit that requires a strategic resource, and then go negative in that resource.
That is, you lose a source of Iron that puts you below 0 Iron, all units that use Iron will have a -50% penalty until you get more iron or lose some units.
This is especially annoying with Aluminum and Uranium if you have puppet cities that use those for buildings. If you get in a war and temporarily go negative, all of your nice units become expensive rubble.
As per your original question:
I ignore musketmen in general. It's not worth the switch if I can have a unit with more base defense. that, and it's a pretty easy jump into rifling at that point.
The point of the musketmen is in situations where you've got no iron available but need units. They'll do in a pinch, since horse-based units are easily countered with cheap pikes or spearmen.
The french and the americans have unique units instead of musketmen, so they are more likely worth it.
Best Answer
The amount is unlimited. You can have level 15 warriors in the ancient era, in theory.
You might have been confused by the fact that a unit with 30 XP or more cannot gain any XP from barbarians... but this is independent of era.