If you want to just stand there and let the water jet do the work, you need something reasonably powerful and around 5 litres of water per bike and I don’t think a handpump one will clean the bike sufficiently when the bike is filthy and the mud is sticky.
But if you don’t mind getting stuck in with a brush and just use water to loosen the mud then you can easily get away with a hand pump and 2.5 litres of water even with a totally covered bike.
I have a washer similar to this washer.
I use a plastic jerry can to hold the water and use a spare 12v battery I have as the power source you can also use the car cigarette lighter socket.
Its not as powerful as a Nomad washer but if I attack the hard bits with the brush then I have no problems cleaning two filthy bikes with 5 litres of water.