I just found a 'unowned' Horse in the wilderness.

How do I keep it?

I did'nt steal it and I have a hearthfire house with stable, how do I keep the horse so it doesn't run away?

How many horses can I own?

I don't think you can keep that horse, it'll follow you around if you mount it but it will eventually stop. That's what happened to me anyways. Also, you can just get your own horse from your steward. To answer your second question, I think you can have as many horses as you like.