I routinely wash 8x10 prints in my Washmaster Eco and have never needed to use a clip. The prints tend to float near the top of the slot and are easily removed.
As to your second question, I'm not sure it's a good idea to put more than one print in a given slot so I would say you can theoretically wash up to five prints at once. However, Nova recommends that you insert the print with the emulsion side facing the textured surface of the slot divider, but the last slot has no textured side. So I would say the washer only really supports washing up to four prints at a time.