Maximum number of outstanding loadLeaderboardsWithCompletionHandler: from a single iOS client?

Question

I am trying to fill a table that shows leaderboard sets along with their titles and numbers of contained leaderboards. (Similar to what the Game Center app also does.)

In order to fill the table, I send off many loadLeaderboardsWithCompletionHandler: requests to instances of GKLeaderboard in parallel. It appears that for the first dozen or so parallel calls the completion handler receives expected values, but for later ones it receives nil for both leaderboards and error.

Could it be that there is a maximum number of parallel loadLeaderboardsWithCompletionHandler: requests that may be outstanding (from a single client), and are specific limitations of this kind documented by Apple?

Answer 1

I've tried combinations of the following remedies:

1. throttle to only one or few parallel requests
2. enforce delay between requests
3. repeat requests for a number of times until leaderboards is no longer nil

A combination of throttling to only 1 "parallel" request and repeating requests up to 2 times proved effective for a set of approx. 20 leaderboards.