If it was me I would most likely do 4 possibly 5 plants tops. Anymore than that and no room to grow and worst yet no way to get to all of them because you have to fit in there also to tend them I have an 8x12 room that I built and tops will be 21 plants and it gonna be tight to get myself in there to tend.
As far as the 40 amp breaker, I read somewhere to plan on light taking 15 tops on its own. Yet you have a few extra bulbs Depending how elaborate this room is gonna be, when I built mine I put in a subpanel that runs off a 50 amp from the main box then the sub has 6 seperate 15 amp breakers in it. I also put in a GFI outlet that anything that plays with water gets plugged into. 40 amp can probably handle it but remember that if something trips the breaker you lose juice to all. Oh and dont forget ventilation :thumbsup: