How many depends on the size of your sav and the size of the roach. Roaches start off pretty small but growth up to three inches long.
If you sav is fifteen inches or less long, I would try four to six one to one and a half inch long roaches. I find that lizards pretty much ignore the roaches they don't want after they are full. Even though savs may each too much sometimes, if they are like my tegu, they will each a few roaches then quit.
Unfortunately, I did not have roaches when I had a sav. And, my albig ( at 68 inches and 29lbs ) just ignores them.
The roaches eat a lot of things, especially fruit and especially tropical fruit. I would recommend a steady diet of fine ground chicken mash, which can be purchased at a pet or feed store fairly cheaply. Also, give them a water bowl (small) with rocks in it (so the little ones dont fall in and drown). Temps should be around 85 to 90 degrees constant with a very high humidity.
Good luck. CW