Cw0rk is totally right! There was such rule. And yes, starters were around 25 cards per side, if I remember well they had a total of 60 cards including sitepath, ringbearer and ring (11 cards), so the remaining 49 were distributed among FP and Shadow (24/25)...
Anyway, it's much easier to add 5 cards to the Shadow side (30/30) than to remove 5 to the FP (25/25). But it's true too that adding 5 cards to the Shadow side is much easier than to add 15 to the Shadow and 10 to the FP (40/40). So the goal is now much simpler.
Thank you Cw0rk!