I found one for $200 near me to buy. My commute is about 13 miles each way. It's a MTB bike instead of a road bike or hybrid so would it be decent for this and how many gears does it have?

Just restored/overhauled one of those for a friend to use as an all purpose utility and short haul commuter.
Took it out and rode the heck out of it before turning it over and was surprised at what a decent riding bike it was.
On the down side, that is a pretty solid frame which is great for a utility bike but a bit stiff for longer commutes. With a set of street tires you could do it but you may find the ride a bit harsh.
Stock they are 21 speeds but with tiny chain rings for ground clearance. Great for hill climbs with a load but you will run out of legs on the downhills.
$200 is a fair "great condition" price. If it needs any work, talk him down or walk away.

They made these for a number of years and now they make a current model too so there is only so much we can really tell you based on the information you gave us. It won't have more than 21 speeds unless someone has modified it. $200 seems like alot unless its totally ready to ride or one of the newer models and they were pretty cheap too I believe so they probably weren't to much more than that to begin with. You might end up being more happy with a hybrid, but mountain bikes can be great for what you are wanting to do depending on alot of factors which we don't know either except your length of commute.