I have 5,6,8. can't count this early. I just put one of those cheap dollar store window mesh drain protectors in the sink, and using the sprayer give everything one at a time a good soaking from the top. the leaves, branches, trunks love it. learn to do it gentle but full enough to get the job done, and you don't lose any bonsai soil. move it to the drain side and start another one. let that one drain also. put those two back and start all over. I do it in between commercials on tv. . sk.
Lowes & HD have these cheap plastic trays that you can pick up in a number of sizes. And I also have a super tiny watering can that makes it super easy to control/avoid spillage.
This was part of my setup from last winter. The pots on the left and right are in 10x20 trays (standard seed starting flats) without holes. The tray on the left has a cut down section of "egg crate" plastic - the white grids that go under ceiling flourescent lights.
My biggest problem was emptying the trays when they got too full. I thought I'd be able to siphon, but it was tricky to get it started in such shallow water. I basically took to moving the pots off of trays, then carefully dumping the trays outside (the shelves were right by my back door.)
This year I plan on adding drain holes with plugs so that at least I can drain them into a bucket.