I use BG v3s and Pop-in bamboos, and I do always boost the BGs now, though I didn't need to when Adam was younger. I add a Little Lamb bamboo booster underneath the microfibre insert, sometimes I fold it in half and place it up front. I don't use the newborn inserts any more, but you could use them both (new BG V3s come with the normal and newborn inserts). I also sometimes boost with hemp (Easy Peasy boosters) if I think the nappy might need to last a lot longer than usual (if we're going out for example).
I don't boost the Pop-in over and above the two snap-in inserts. I do have the night booster but found the whole thing was ridiculously big, so I use it with a Flip wrap during the day. It's a little short but Adam wees all up front so it's ok.
I'd recommend some bamboo boosters and some hemp (very absorbant and very slim). Lots of sites sell them. Little Lambs are cheap. P'tit Dessous are the most prized, from what I've read elsewhere, but they're more expensive. Easy Peasy does good hemp boosters. How many depends on how many nappies you have that might need a booster - I have about 15 altogether, and have never run short, but we don't boost everything, plus we don't use cloth overnight. If we were I'd say that was probably an ok amount, I might actually get a few more.
You can boost with other stuff in an emergency - a face cloth, for example, or a microfibre cloth, just a regular household one.