4 cups is probably about the limit for a single session, based on my use of a very similar machine. You need to be very careful that you dont let the Boiler run dry though; after completing a steaming session you should refill the boiler by opening the Steam Valve and hitting the Hot Water Switch until such time as you get a continuous stream coming from the nozzle.
If you need to make more coffees after the previous 4 then just repeat the routine until all coffees are made. After refilling the Boiler (should do this after every coffee making session), it probably takes about 5 minutes for the Boiler to recover and be ready for brewing again. As for trying to brew more coffee immediately after texturing milk, this is not a good thing to do for the reason you mention and also because the Boiler water level will be low... always need to top up the boiler after steaming.
All the best, :)