Saulosi's are typically found in stores which have a wide selection available (though this may vary from area to area).
In a 55gal you could probably stock 8-10 mbunas (such as 4-5 saulosi's and an identical quantity of yellow labs). You could probably stock as many as 15 mbunas in a 55gal (which is often cited as the minimum sized tank to keep mbuna's in).
Some have had luck keeping yellow labs in smaller tanks, so maybe 5 in a 30gal (1m/4f), though the 55gal is recommended over the 30gal.