Depends on how big you want them to get, really. Here's a couple of pics. The first pic is three Indica-dominant plants (White Rhinos on the sides, AK-48 in the middle) that were vegged for around 10 weeks (had to leave town, didn't want to flower until I got back). The fact that the ScroG screen went basically unused in the first pic is because of that long veg time -- the plants just exploded when they went into flowering and I had to fight them all the way through flowering. That pic was taken pretty late in flowering, day 45 or 50 I'd guess.

The second pic is a Sativa-dominant plant (Durban Poison x Skunk #1) that was vegged for six MONTHS (but most of that time in a 1/2 gallon pot, which limited its growth), and the pic is after 9 days of flowering (which means that plant will grow at least another foot or two, maybe more, maybe a lot more). The cabinet in the first pic is 20 inches deep, 47 inches wide, 6 feet tall.

Your closet would fit two plants like those in the first pic, but might not handle even one plant like the one in the second pic.