I agree with mjs: start with two sheets and increase as your skill level increases. I like doing four at a time, but I'm still on the upslope of my learning curve.
There are some good videos on YouTube (type in "processing sheet film in trays"). I prefer 5x7 trays. I find the film moves around with the rocking in anything larger, and I lose track of the order not to mention the risk of scratching while trying to gather them up. With a 5x7 tray, I can easily collect the pile at the end of the tray and pick the bottom sheet out. I put the notch codes of the bottom sheet at the top and flip all the other sheets 180 degrees so I know how much each sheet has been shuffled.
My last suggestion is to practice in daylight with wasted film. When I bought my used film holders, there was film in them. I saved them for practicing.