There's no limit as of yet but please make sure the posts go according to our curation rules (niche's we don't curate can be seen in the OCD community sidebar) and to attempt to curate underrewarded authors that post original content. You can still get rewarded for your tweet even if the post you nominate doesn't receive curation for one thing or another as long as you tweet well (use proper tags, etc) to try to get as big of a reach as possible.