First of all welcome to AG and congrats on the purchase.
As for pads, well I have both the Lake Country 6" pads and the Edge 2000 6.5" pads. Both pads are great, I like the Edge pads because they are two pads in one and that they are automatically centered due to there adaptor that goes on your machine. If you have a rotary, you will have to purchase the rotary adaptor, not the DA adaptor.
AG has great selection but I would encourage you to look at a package, you can get the Edge pads plus other goodies for a really good price. If your doing a complete detail I guess it depends on the vehicle but I would at least get orange pads, green pads and white pad...at least.
As to how many, I got two sets but one set will do just fine. If they get too dirty to use simply clean, spin dry and away you go. I like having two sets because I can simply remove the dirty one and install a new clean pad and away I go.
Hope this helps.