My Regirock had max HP EVs, but when I used Pomeg Berries on it, it took 20 (I counted). I retested this on some other Pokemon. Did they change the amount berries, because I thought that berries reduced the EV to 100 if it was above, and then 10 each other time (this means it will always take 11 or less berries).
This Berry increases a Pokémon's friendship by and lowers its HP EVs by ten points (but not below 0). It can only be used outside of battle, and only has this effect in Pokémon Emerald and later games. -Bulbapedia
Pokémon are limited to a total of 255 effort points per stat, and 510 effort points in total. -Bulbapedia
Pomeg Berries have always lowered HP by ten. They don't reduce the amount of H EVsP to 100 if a Pokemon has more than 100 EVs in HP, so, if a Pokemon has max HP EVs, it would take 26 (25.5, but you can't have half of a berry) Pomeg Berries to reduce a Pokemon's HP EVs to zero.