How many passes? Let's see... how thick did you start at? How thin did you want? However, most important... How's your power supply? In my shop, I know right away if I try to take too big a bite... the circuit breaker trips.
Power aside, I really get into grinding lams. I do 'em one to a pass, have them all lined up and get into a rhythm... start one through, step past the sander to catch it, mike it, then do the next one, etc. I really do think you get better control if you go real thin when you're at the point of getting serious.
I'll use a full crank, or turn, of the handle when I start, to get rid of the saw marks, but quickly go to 1/2 turn as I get smooth, and work with 1/4 turns as I close in on it.
Always mike! While we'd like to pretend that these things are precision machine tools, they ain't! They're wood working tools. Sandpaper wears and clogs. It gets thinner in the middle if you run most of your work there. Get to know your machine and how it's performing. I'll sometimes mike a lam and decide that I want just a little more off and decide to send it through for another pass without lowering the drum any. I'll send it through in the middle if I want a thinner pass or over toward the edge, where the paper is maybe newer if I want a thicker pass.
Also, don't forget to play with the lam orientation... mike and flip, mike and end for end. Find out how to get the flattest, truest lam. Like I said, you have to get to know your machine.