The maximum healers per group will be 3. Although, if there are no healers in queue the BG will begin without them or if they are more, the each team will not have more than 3. Also, there is a possibility that the BG will be played with 2 healers or 1 for each team in case there won't be other healers in queue or if all the opened BG spots were filled with DPS classes.
In few words, the new match making system will try to offer equal amount of healers for both team. Of course, it won't be bulletproof because a healer who is already in BG, he might go afk or leave the BG, or even skip the BG queue because he will decide to do other activity, so the balance regarding healers will change.