I've got three cats and three litter trays but all the cats insist on using the same one and seem to save the other two for dire emergencies So I change the main one at least every other day and sometimes daily, and the other two only need changing once a week, so infrequently are they used ....
Mine do that too! I have 9 trays for 10 cats (oops, not quite the 1 per + 1 but near enough LOL!) 4 of the trays are in a walk in cupboard off the kitchen, 1 in dining room, 2 in 2 different upstairs rooms, plus 2 in the garage. The most used trays are the 2 at the far end of the walk in cupboard off the kitchen - they prefer those even if they've already been used and the other 2 in the cupboard are clean - go figure!
I use clumping litter to about 2-3 inches deep, in open trays, scoop poops several times a day and take out clumps several times a day too. Empty each week and wash trays with washing up liquid and hot water, spray with Milton, refill with any remaining clean litter plus top up with new. Run the trays right down once a month, dump and start again. Between the weekly wash and disnfect I wipe around the inside and lip of trays with Milton.