Depends on your category, but I'll use most NB categories which have 3-99-5. What this means to you is you can have no fewer than 3 consecutive on-call days at a time and you can have no more than 5 groupings of off days. The 99 is n/a for any realistic schedule so you can ignore it. For reference, check out the first few pages of any bid package which will explain these rules in depth.