One probe, but a 500 watt output capacity.. I use one per rack. and each rack has 18 trays. Not every tray will be the exact same temp, as the upper trays in racks will usually be hotter, but you can put the probe about half way up, and split the difference. You basically use them to set your hot end anyway, so a degree or so difference is not big deal
I use open back racks with flexwatt under the backs of sliding trays, for gravid boids. I simply adust the heat tape so that it is further back and not as close to the tray on the top ones.. this is an easy way to adjust for the upper trays being a bit hotter.
Here's a product link...