To be fair, there are clearly the plates set on the other side. The painting is supposed to catch the moment when some of the apostoles stood up to get together with their comrades. you can see some of them standing behind the back of the others, not sitting.
Also, the apostole on the Jesus's right is clearly a woman...
No, they did that because Jesus had just broken the news that he'll be betrayed by Judas. The whole scene shows the shock and a heated discussion of that revealation.
Also, the apostole on the Jesus's right is clearly a woman...