Since there is inexplicably no stickied resource for this, and I thought there was.
CWB (clan war belt) rules are the most common online ruleset for RTW gameplay.
They are:
-Max 6 of any one individual unit type
-Max 8 cavalry (e.g. in keeping with the above rule, for Rome this may consist of 6 Praetorian cavalry and 2 Legionary cavalry)
-Max 8 long range foot missile types (slingers/archers)
-Max 2 horse archers (These count towards the cavalry maximum, but not towards the missile maximum. That means that a faction like Scythia under these rules could contain 10 archer units when the 2 horse archers are included)
-Max 2 beserkers
-No artillery
-No elephants (The lack of artillery and elephants is not because they provide an "unfair advantage", but precisely the opposite, to discourage new players from using ineffective units that are easily countered)
-No red line camping (Again, this is mostly about new players trying to create bottlenecks and unflankable positions which will be ineffective against an experienced player for the most part)
-No faction reselection (This is not a rule that is necessarily always adhered to)
*NB: Peltasts, skirmisher warbands, velites, militia cavalry and any other javelin armed units do not count towards any missile troop maximum, which is strictly reserved for foot archer and slinger troops.
*NB: A unit of chariots is considered as a cavalry unit and as such is subject towards the same limitations as just another cavalry unit.