We used the MotherEase Rikki wrap covers, but I would have preferred to have the AirFlow snap ones instead. The velcro was very noisy when I was changing her in the middle of the night and always woke her up - she'd jump at hearing the riiiiiipppp sound. LOL
I bought a lot of them from Craigslist and got 3 covers and 20 fitteds. This was enough to wash every 1.5 days so I'd probably want a few more to go a full 2 days. 30 would probably be a nice round number to not have to worry about running out but since you'll be using prefolds too I'd just get a mix to add up to that many. Though I will say that I could have probably done with only 2 covers (though most people recommend 4-6 for a newborn). Because the fitteds really held everything in I only had like 2 or 3 times that some of the poop got on the cover, so 2 covers was enough to have one on the baby and the other airing out after use. Not sure how good the containment with prefolds is though so you may need more for that.
With the XS Sandys I also tried the Econobum and Flip covers that I had, just to see if they would work and they did. I don't like the fit over my size Small Sandys and the MEOS fitteds that we're using now though. I also got some fleece covers which I quite liked and they were nice and soft. I recommend snap-on ones as it's tricky to pull them up on a newborn (and messy to pull down in case of a poop-splosion). Mine were these: