5 PLANTS IN 7L POT OR 2 PLANTS IN 18L POT?

Yeah I'd go with 2 plants under that light . You would need two of those lights to get a decent yield off 5 plants. I like to do less plants and have bigger pots myself. Bigger roots, bigger fruits .

you don't have enough light for five plants in my opinion.. so flower them sooner rather then later you will thank yourself later in that all five will be produce. if you let them get to big they will be fighting for light... And yes bigger plants will produce bigger yields but Veg time is a thing if you have a big container you would want to veg longer...