Don't hide the fact from them. Here's an example from 2 nights ago...I picked up this spanish girl(who's number I got from a bar a couple of days before). As we're driving around, I'm doing the usual C+F routine...and she asks me "So you must have a girlfriend?" I said "Yeah, I've got like 4 or 5". There was a pause for a second before she said "Are you serious?" to which I replied "Yeah, but they're not exclusive!".
Arrogance works wonders my friend. 10 minutes later I was porking her by the side of the road while cars were driving by and probably getting a good look at my butt while I was nailing her.
I actually do have about 4 or 5 girls in rotation all of the time. I find it gets a little tricky trying to fit them all in though. And since about half of them just want to bang, and half want something more serious, it's difficult to schedule sometimes.
I'd take it slowly at first. Start by juggling 2, then work your way up. I'm actually thinking of reducing the number I have to 3 at the most. 5 is just too much and I end up not having time for one or two of them.
Also, don't get lazy! Keep getting new number and adding fresh girls into your rotation, and dropping contact with girls that are beginning to either bug you, or bore you.
I find for me, I will keep a girl in rotation for an average of 2 or 3 months. Seeing her maybe once a week, or every two weeks. This should allow you see 5 girls without causing TOO much double-bookings.
One last thing, and undoubtedly the most important: Don't forget your friends! Man I can't stress this shyt enough. Let ALL of the girls in your rotation know that you party with your friends often, and will do so whenever the hell you feel like it. Bros before hoes, man. Your friends serve 2 purposes. 1)They're your friends, enough said. 2)They're how you'll be getting all of these new girls for your rotation. Most of the girls I pick up are during nights out with the boys.
Good luck.