Does it matter? Plan on getting it back to the charger or up on the trailer before it runs out of juice.
Just like you refill your tractor before it runs out of diesel.
Or if you have cartridge batteries, have a couple to spare.
Pay attention to how the mower sucks juice out of the cartridges. On my old Greenworks push mower, it would drain one battery then move on to the next. So, it would run 100% on a single battery. I don't know if the riders do the same, but I'd expect that it would be able to at least drive on 1 or 2 battery packs.