I have used one for daytime and two for outings or sleeping. It also depends on which microfiber towels you buy. I've found that the larger yellow ones from sam's club and cosco work better than the smaller blue and white ones you can find at target and walmart. They end up being bulkier than hemp inserts but they dry in 1/4 of the time.
We use 2 for daytime and 3 for night on my frequent wetter. They ARE a tad bulkier than hemp inserts but they really do dry ten times faster. It's worth that tiny bit of extra bulk unless you have a TON of those expensive hemp insrts that you can rotate around to allow for the extra dry time.
I use 1 tri-folded for day. My "regular" inserts are 2-layer hemp fleece doublers over cotton contours (I have a ton left from DD2 and DS2's diapering days that I have to use or feel guilty) I also use 2-layer trifold hemp fleece inserts. They're all about the same for absorbancy, but I don't think the micro holds as well as the hemp - the diaper inself seems wetter when I use just micro. I just started using hemp/micro/flannel inserts that work really well with the hemp backing up the micro.