Is there a limit on how many shares a user can belong to?

I know a CKShare can only have 100 participants. But I’m asking the other way around. Is there a limit on how many CKShares a user can be a participant on?

The idea is for each user to have a shared zone with my service, that would act like an inbox. So my service would be a participant on each user's CKShare, and if the app is successful then server could be potentially participating in thousands of shares.