How Many Posts by a Member Can Be Front-Paged in a Day?

There is no formal limit on this, but it would be unusual for this number to be very high. At the time this answer is being written, there is enough good content being posted to the site that it is impossible to get it all on the front page (and still have the front page be useful). And thanks to "track changes" and the recommended list, which both allow visitors to find good content that is not on the front page, there is only limited pressure to put everything good on the front page.