I found a stray dog i heard they are random so will i have a chance to collect em?

I'd imagine you can only have one dog as a follower at a time. Mind you, I'm not certain, if you could have a wolfpack following you round that was be awesome.

You can have one normal follower dog and another quest dog long as you dont complete the quest (no spoilers) plus a 2 legged companion.

So you can have a party of 4 including you. However because the dungeons have such narrow corridors they will probably be more a hinderance then help.