2 or 3. Personally I had better luck with 2 and still got roughly the same yield as when I grew 3. I found with 3 plants in there they were overlapping each other and the air flow wasn’t the best. But it all depends on how often you want to crop out. You can do more plants and harvest sooner or less plants and have a later harvest. I’d be surprised if you hauled a pound from that tent though
I usually run two pots in a 2x4. Upsize to two 10 gal pots and you'll be able to pull more than a pound off of the 2. You can totally run 3 or even 4 but it'll eventually get cramped in there.
In a 2'x4' with a 5-gallon pot, I'd go with 1 plant and LST it. But understand, mine get wide, bushy and lots of colas. One plant would fill that sized tent for me. A pound is being overly optimistic for the way I grow though. YMMV
I'm running a 3'x4' right now and at 9 weeks from seed the 2 autos in 5-gal. pots have filled the tent and are overlapping, so that's why I'd run 1 plant in a 2'x4'.