Labs dont get 6" held in captivity. You are looking at 4" average (5" max), IME.
The amount depends on the water quality and parameters. Remember, they like hard water and high ph. If you have excellent filtration and do 30%-40% weekly water changes, you can have quite a few (6-8), and they dont have to be cichlid only, however it is preferable. Labs are very docile for african cichlids - I have kept them with other fish that were rescued with no cause for concern.
Now if you are new to fishkeeping, dont do weekly water changes and your parameters are good enough (high KH/GH and high PH), that number will come down.
Labs are very durable and I have had up to 8 in a 29G lab breeding tank for years (about 4 years) with absolutly no problems (dual filters, 40% weekly water change, GH=24, KH=25,ph=8.1). They breed like crazy and then I ended up giving them to a good home (16 labs all together). All were very healthy, no disease, no stress and they were good parents, once they got the hang of things.
Also a note about aggression and cichlids. Many people like to overstock the aggressive africans to spread out the picking on. I do this in my 55G and have not had any problems for 5 years (Kenyii, Zebras, RTS [not a cichlid] and YTV Cichlids). Yellow labs "really" dont apply to this rule though.
Soooo....I would start with a few (ie 4) and move from there depending on your setup, water and your skill.
Good Luck
Aries