As
@raindownthunda said, the amp is able to comfortably handle 2x 6080/6AS7/5998 per channel, and can do 1x 6336/6528 per channel with the optional 6336/6528 handling upgrade. When you get into 3x 6080/6AS7/5998 per channel, the amp gets into inferno heat territory and the lifespan of internal parts starts to really suffer, so 2x 6080/6AS7/5998 per channel is the max I recommend for running long term.
EL34 tubes only take 60% of the heater current than 6080/6AS7/5998 though, so those can run 3x per channel in adapters.
6BX7/6BL7 on the other hand need to be run 3x per channel when running the amp in constant current sink (CCS) power tube bias mode, but it's possible to run as little as 1x 6BX7/6BL7 per channel when using the resistor (RES) power tube bias mode.
Edit: I'll give a more comprehensive technical background of what is happening with all the different modes of the amp in the next few days when I get some time off work. It's some neat technical elements that allows the amp to be kind of like three somewhat different kinds of amp all wrapped into one and swappable with flicking some switches on the back (with amp off).