Every time I count cells, I not only write down the cell density (likely most relevant for the transfection i’m about to do), but I also write down the total volume of cells and the vessel the cells came from. Thus, I’ve essentially figured out how many total cells there were in the plate I was trypsinizing. I’ve mostly done this for T75 flasks, but I also have a handful of counts from 10cm plates as well:

So, in short, T75 flasks more or less max out around 20 million cells (hence the peak there, but the “confluent” flask), although I’ve gotten some larger counts before (maybe really packed in there, or maybe the result of counting error). 10cm, despite being slightly lower in surface area, has comparable counts, but that’s likely b/c I’ve tended to have consistently higher cell densities in there, since I’m usually doing end-point experiments in those and doing more routine passaging in T75s.