I will likely play one, maybe two even. It depends on the deck, my Darkrai/Garb/Tina deck is all basic EX attackers, Garb gets around carbink, but that space that I would normally use for Hex maniac in my other decks will be a pokemon ranger. I also am considering running a Hydreigon or Tyrantrum who can attack through Jolteon with ease.
Hydriegon with FFB hits for 90 and has 220 HP, Jolteon with FFB swings for 80 and has 200 HP meaning we three shot each other, but if neither of us have FFB then Hydreigon two shots Jolteon but Jolteon still has to three shot Hydreigon. If you have a FFB but I don't, then we still three shot each other. So advantage Hydreigon, the match will likely come down to who can force their attack first. (assuming Jolteon already attacked with flash ray, if not then I would lysandre/escape rope combo and OHKO with Darkrai.
I think a 1/1 split, as Jolteon isn't the only pokemon who will be around who Ranger will need to deal with. This is assuming standard of course, in expanded I will almost certainly be playing two, and battle compressors to get them out as there is sooooo friggen much in expanded Ranger shuts down. They will be as useful as Lysandres I believe. Toad, Tina, Jolteon, Glaceon and a dozen other good cards in expanded all die to Ranger.