There is no limit.
Pokémon LV.X cards also share the same name as the Pokémon they Level-Up from, and as such only four of either card are allowed in a 40- or 60-card deck. For instance, a player would be allowed to have two Honchkrow LV.42 and two Honchkrow LV.X in the same deck, but only four Honchkrow would be allowed in total. As well as that, because of the fact that these two cards share the same name, an effect which would remove the highest Stage Evolution card from a Pokémon would, in this example, force the removal of both Honchkrow cards, leaving only Murkrow.
It's limited by the name. If you have 3 Mews and a Mew LVX, no more Mews are allowed. That's all.