Right now I have ED-E, which I just got, and Boone with me. I also have Rex, but he's in my hotel room as of now. 
So, who do you got covering your back?

ED-E and Boone, obviously.

Working on ED-E, but got Boone for now.

Im playing on hardcore and so far have kept Boone.

ED-E  and  veronica

cass (for a while) now lily and rex

ED-E and Boone. Love both their buffs to my character.

Veronica and ED-D. I don't like V's perk too much but she's good for Close Quarters and ED-D is helpful do to my somewhat low perception.

Raul (with bitchin vasquero gear), and ED-E is somewhere, but it got lost and I dunno how to get it back :(

I haven't sought out a companion yet, but I guess that answers one of my questions, you can have multiple companions then? Damn---that stupid achievement probably is as insane as it sounds, you probably have to have ALL of them alive and with you...if so, that's all kinds of ridiculous--- lol

I would have ED-E, but I'm playing on hardcore mode on very hard and he was destroyed very early on in my game.  Right now i am finishing Cass' quest, so she and Rex are with me, but I will probably switch back to Boone when her quest line is over.  

i had boone and rex, but i wanted to help caesar's legion and boone kept killing them. i got rid of him for now.

I don't understand how you guys can have multiple companions :S. Don't know if it is that i'm only lvl 10/11 or in the beginning/mid stage of the game, but I can only have one, I have Veronica with me now(a really kick ass and helpful companion in battle btw) and tried picking up Cassidy with me and it said I have to many companions with me :S.....

I had Boone but i had to ditch him since he wont fight those NCR Bastards. 
 
Have to admit though, he made the game a cakewalk for me.

I cycle between Arcade and Cass (Rex is always with me). The game is pretty easy so I don't' worry about having the most effective companion, just the ones I like the most.

Rockin ED-E and Veronica right now.  If you give Veronica a Ballistic Fist, she straight up one/2 shots almost anything.

I usually don't use companions, but Boone is a beast with a sniper rifle. By the time I've zeroed in on an enemy he's already taken their head off.

" ED-E and Boone, obviously. "

Right now I only have Cass with me and she's pretty good, haven't found anyone else
 
A question though, how many companions can you have with you at a time?

I mostly used Veronica and ED-E on my first playthrough.

Boone and robot dog. 

Haven't played enough to get any yet, but I plan on getting Arcade and the dog, probably.
 
Arcade is swell.

" Right now I only have Cass with me and she's pretty good, haven't found anyone else  A question though, how many companions can you have with you at a time? "

I used Boone and E-DE a lot in the beginning but changed to Rex and Lily about half way through just because they are way cooler. Foolishly I changed back to Boone just before the last mission and he got pissed off and left after I turned on the NCR.

E-DE and Boone for now. Gonna change Boone for Raul later on.

I had Veronica with me but she disappeared and now I can't recruit anyone else.