Is the amount of followers limited?
I have one Mage follower right now, and he fights well and holds a lot. But can I have another companion?
You can only have one follower at a time: if you want a different one, you need to dismiss the first one before you recruit them.
There is one exception to this: there are situations when you already have a follower when a person needs to join you for something quest-related. In this case, the quest-related follower might join you in addition to your current follower. However, there are situations where he or she will just replace your current one instead.
At any rate, this is temporary, and the second follower will leave when the quest is complete.
Followers can be found all over the world: it'll be hard to avoid finding followers. There are 45 in total:
In addition to a single follower, you can buy a wardog in Markarth Side for 500 gold, which acts as another member of your team. There are two dogs you can choose from: Vigilance and Meeko. You can also pick up a stray dog after a random encounter.
Finally, while they're not followers, the Conjuration school naturally gives you access to spells that summon the forces of Oblivion fight for you for a period of time (or permanently, at the higher levels).
You can have 3 as a baseline, with the extra's coming from abusing the conjuration skill.
Theoretically, you could get up to 14, which requires 2 Master Necromancer Thralls and your conjuration follower to raise their own conjuration-using zombies. Hell with chain raising, your party size is only limited by time and number of conjurer corpses you have available.
So in summary, 5 or 6 permanent followers, with an additional 4-to-infinity available temporarily in battle.
Note: In dungeon tunnels it gets very crowded. If you don't use a follower, you drop down by 2, but everyone in the party is expendable. Go Ice Storm happy ;)
Here's a video showing how to have 3 human followers, a dog, and spirits with you:
Partially strange but currently in the game I have two followers, one mage that I had hired and Mercer since he was lazy and wouldn't open the darn cave door, so now I just wander the country side with them completing miscellaneous tasks for the time being. So I would say its possible to have more that one follower that is human.
I was somehow able to get two human followers (non-quest). One acts as a "normal" follower, which in this case is my housecarl from Solitude, but the other which is Cicero is a regular follower with the strange exception of being one that ignores the fact that I already have one following me around. I know its Cicero and not the other one because when I try to add another follower he/she will refuse saying that I already have a follower UNLESS I dismiss my housecarl, which allows me to pick up a new one. Now, before you ask yes I finished the entire Dark Brotherhood questline and yes I can customize his equipment and give him orders. And no I haven't dismissed him yet to see if it would allow me to pick him up again.