i havent been busted (touch wood!!) but ive heard about peoples who have, and it really depends on a number of different factors as to what punishment you get, in my mind this is what i consider to be a rough guide:

severe punishment (whatever that might be, prison, hefty fine etc) would be gained from growing anything over 10 plants or even less (im not too sure!) in a professional way eg hydro hps setup.

lesser punishment from growing a relativly small amount eg 3-6 plants because it is obvious your not growing to supply large demands the law sees it as growing for personal use, but still if the police are being bastards they can get you in alot of shit!! (as a mate found out!!) expect to get in some trouble for this amount so be warned.

slap on the wrists type punishment would be growing say 1 plant say outside sumwhere as long as it doesnt get huge!!

as i say this is what i believe to be roughly the score but as with most things i am probably way off!!

as a first time grower you need to suss out how much risk of being busted is there then make the decision on how many plants is safe to grow!!

good luck!
P.S. i fukin agree bout the weed in the uk at the moment, theres too much crap about, nice stuff from local grows about a bit more now but still pricey!! £25 on the bloddy 1/8 nows the time to grow your own!!