"If you are just thinning on top, 2-3 treatments will suffice. If you are receding and/or completely bald on top your looking at 5-8 treatments".
I'm afraid that advice is quite inaccurate and very dated and the type of sale spiel they tell you in the "hair mills" (the McDonald's of HT clinics). No offence but reasearch is key.
Megasessions now-a-days can restore a high Norwood (balding scale) in one session. Maybe a 2nd to to add density and refine and soften the hairline. Good doctors require skill and enough technicians to disect the graphs on the day in-order to plant 2000 - 7000 + grafts in one sitting. Anytthing less than 2000 grafts i would look into "FUE" Follicular Unit Extraction.... as there is no strip cut out and it's minor surgery. Expensive, but for that volume required it's not really worth slicing your head open.
Inferior clinics don't have the facilities or staff to do these sessions so they will have you back and slice you open again and again, taking more money each time. They commit you to a expensive and traumatic lifelong treadmill of repeat surgerys and it's totally un-necessary these days.
5 - 8 treatments

..... this is where the problems occur. Scar tissue, trauma, permanent nerve damage, shockloss of the existing hair !!! Every time you cut into skin, the tissue is damaged, skin is removed and scar tissue forms. Scalp laxity is lost resulting in scar stretching and you are more likely to end up with a big ugly problem scar ear to ear. You need a good surgeon who has the skill and the staff to achieve the results in 1 or 2 passes otherwise walk away.
"From what I understand you should get the treatments early as soon as you're starting to lose your hair to minimize cost".
Sorry, very, very bad advice.
Did your mates go to the place that advertises on TV?? If so ... run ! Just try Googling their "medical violations" and legal battles.
You should WAIT until your hairloss has stabalized. Teens forget it. Early 20's major risk. Late 20's dodgy. 30's better idea, 40's much safer.
Sure you would like to start early and never bald, but if you "start early" your hairloss may/will progress and you will be left with a very funny looking island of hair at the front of your head. Incredibly embarrassing and far worse than being bald.
You will spend the rest of your life playing catch up, chasing the receeding hairline with more grafts, spending time and money under the surgeons knife and waiting around 12 months each time under hats waiting for the hair to grow.
Initial surgery costs are negligible compare to repair costs.
Research. Travel to the best possible (most reimburse your airfare) and take a lot of care.