"Republican era Legion
At optimum strength the legion of the Republic had 4,200 men. Each legio was made up of ten subdivisions of 420 men, known as a cohors ("cohort" in English). Each cohors consisted of one maniple of 60 triari, one of 120 princips, one of 120 hastati, and one of 120 velites."

From: http://www.redrampant.com/roma/legiounit.html

Also a good site for legion composition: http://www.roman-empire.net/army/army.html

For Polybian Legions I've always taken that as:

Real Life

Unit % per Legion| Men Per Legion (Real)
Velites 28.57%| Velites 1200
Hastati 28.57%| Hastati 1200
Princips 28.57%| Princips 1200
Triari 14.29%| Triari 600

Total 100.00%| Total 4200

Game

Unit % per Legion| Men Per Legion (Game)
Velites 26.79%| Velites 300 (100x3)
Hastati 29.29% | Hastati 328 (82x4)
Princips 29.29%| Princips 328 (82x4)
Triari 14.64% | Triari 164 (82x2)

Total 100.00% | Total 1120

Tried to keep the % as close to the real life % as I could.

So, 3 units of velites, 4 units of hastati, 4 units of princips, and 2 units of triari. Add a unit of Equites Romani and a General. So a total of 15 units per legion on large unit settings.

Anyway, that is my model for the "core" of a legion. Add extra regional units only as needed.

A large discussion was once held about this topic. It can be found here: https://forums.totalwar.org/vb/showthread.php?t=87658 and https://forums.totalwar.org/vb/showthread.php?t=79935