You will be advised of specific numbers, as appropriate nearer the date that they may visit, however it's not unheard of for someone to drag about a dozen of their family & friends to their passing-out parade.
The initial visit should really be just Mum, Dad, siblings & partner at most- anymore is a bit pointless as you'll feel a bit daft if you invite dozens & then don't make it as far as completing training.
Common sense applies- there are limited facilities & you have to look after them & escort them around the base for the duration of their visit. Great Uncle Whatsit & his mates from the Legion are best waiting until AFTER you pass-out, trust me.
You can bring 2 male friends, and your training team will insist on as many girls as possible, more the better. :*text deleted*: Just joking
At the end of the first week, you'll get a chat with the Seargent about the visit, who is coming and they'll send your family a letter with all the local hotels (if staying the night previous) and travel directions and he'll sort you out some travel warrants to get home, or if you are travelling back with your families, then it will be to get back to lympstone on the sunday.
I think you get 4 return journeys a year, even if you are only going one way, i.e going home with your family in the car and going back down on the train. It will still cost a return travel warrant, even though you are only doing one half of the journey.
I know for all the youg-uns it would be good to see there family after a week as they'll be struggling with homesickness etc, but living up north its around a 6-7hr drive down to Lympstone as is the train.
Chris, if you live up in the North East, you can fly from Exeter airport instead of a train warrant, probably around 1 hours flight time. Or if there are a few of you, you can use one travel warrant and hire a car via ctcrm and all get it in. If you share warrants between 3 of you, that's 12 journeys per year paid for.