I finally decided to go with the Legion so I was just wondering is this like the NCR where there are a decent amount of main storyline quests or just a small amount of them?

Here the http://fallout.wikia.com/wiki/Fallout:_New_Vegas_quests by faction, including side quests and unmarked quests.

Legion has like... 4 sidequest?
And one of them, Cold, Cold Heart is like: Talk to vulpes, talk to a named NCR trooper, done.
If you're going for full Legion playthrough. (Skipping all NCR quests) then there'll be far fewer quests.
I'm playing a female EW character right now that will never take off her Legion armor for any reason what so ever.
The Strip is gonna hate me...

If you don't count sabotaging NCR side quest to Legion favor, not much.

I really enjoy how I was Idolized by both NCR and Legion at some stage.

I'm going to try to do that one. Good to know it can be done.


B)

Far fewer as mentioned, but you have to keep in mind, most of the Mojave is occupied by the NCR during the events of New Vegas. Most of the Legion is East of the Colorado river and therefore not really on the map. All you see in the game are a few forward outposts. I think the idea was that, like valkebus mentioned, you would sabotage and/or pick negative outcomes when doing NCR quests.