My understanding: It will only email folks who last visited AFTER the day you went live with Xenforo 2.2 (on day 14) Pre conversion users before that day are likely "on hold". By default, ONLY users who visited after that date, will get one after 14 days of inactivity.
Example: If you wanted to have it send users from the previous 3 months before the conversion date, one way would be to go to the admin control panel -> users -> batch update users.
Specify the start and end dates that you wish, in the LAST VISITED BETWEEN section. (so 3 months before conversion day, to conversion day)
Click PROCEED to identify how many will be applied
In the next screen you can tell it what you want to do with that group. ie: (X)Enable Activity Summary Email
UPDATE would execute the command, and set their flags for todays date.
So 2 weeks from today, that group would start getting the activity summary updates. (per the daily batch limit you said, as not to overload the ISP and get banned for being a spammer). It will do that many per day, from the above list, until they are all done.
This is my understanding of how it works, and how it worked for my forums. (One of them is a VB4 that I converted 2-3 weeks ago, and they started sending yesterday for the batch update I did)
I'm not customer support, just a customer, so use the advice at your own risk
Good luck!
Mike