For the longest time, I was stuck at 6 (they'd replenish periodically, but never more than 6 at a time). Many other people said that had 12 at that time. Just the other day, I noticed that my main gmail acct suddenly had 50 invites available at once.
Not everyone got 50 (at least not at the same time). I have a second acct that was still at 6 for 2 days after the primary one got the 50 (which is what prompted this question). However, it too has 50 invites now, so it is probable that gmail is giving everyone 50, but is doing it in batches.