My legion consists of:-

Pre-Marian

6 Hastati
6 Pricipes
2 Triarii
2 Equites
2 Funditores (slingers)
1 General's bodyguard

(I play the R:TR Mod 3.1 which allows training of Hastati, Principes and Triarii from the start).

I also use local mercenary units to flesh out my Legion.
Sometimes I will have less Hastati and more Mercenary foot or cav.

Post-Marian

10-12 Legionary Cohorts
2 Legionary Cavalry
2 Archer auxilia
2 auxilia
1 General's Bodyguard

Again, I will add mercenary or locally raised troops as neccessary.

I usually have a smaller force accompany my main legions.
This is several units of foot, 1 unit of cav, velites/missile troops and heavy equipment such as ballistae and onagers. This is my siege/replacement army.

I prefer to keep the composition of my armies as historical as possible.
I don't like to use larger numbers of archers as it makes battles too easy. I like to get close and dirty with my infantry. I love Roman infantry because they have one ability that most other faction infantry don't - Pilum.