For scrims at least 5 mags are recommended, for longer games and milsim you'd need at least 8-10 mags.
But to be honest I wouldn't recommend a gbbr as a first gun. An aeg would be a superior choice as a starting gun, most if not all of the more experianced players would agree.
Aegs require less maintanance and magazines are alot less expensive. Gbbr mags are about 50$ each, where as aeg mags are about 60$ for 8 of them. Also gbbr users generally have a back up gun in the form of an aeg as well as gbbrs can sometimes be finicky.
I highly suggest you take a look at an aeg as your starter gun.
Edit; I suggest you watch this, it will give you some basic info and is entertaining.
‪Tactical Episode 01 - "9 Things You Should Know About Grunts" (HD) - Redwolf Airsoft - RWTV‬‏ - YouTube