As EGSC said use the guide to find out how many pax per lav although among of crew will vary from flight to flight and carrier.
Havana is correct on that each "cart" will stock different amounts of tray sizes. For information each full size cart has 12 levels plus the bottom allowing for 13 trays if they were to be a full cart length which they are not otherwise a tray would be twice the size as a table on board. The general number is 26/39/52