Sound mature bulls 2+ yr old bulls should be able to service 30 - 40 cows. IMO, too many bulls is worse than too few. Too many bulls, especially young bulls, are injured fighting over one cow and then are out at least for the rest of the season or permanently.
Personally, I would pull the bulls out after 60 days, test and cull any open cows. Leaving bulls in with the cows year round only leads to calving year round. Which is the opposite of what you want to do if your goals are to enhance both maternal fertility and "peas-in-a-pod" uniformity in your calves.
This winter weather many are experiencing should be a reality check to anyone considering leaving bulls in year round. -50 wind chills with snow on the ground is just NOT my idea of pleasant calving weather. Even with the current -10 nights are mostly calm and pleasant, a calf could still wilt down if he didn't get up and going right away
The last few years I owned cattle I moved calving to late April - early May calving start. Branded in late June/early July and doped calves for flies at same time. Doped flies again in Sept at fall shot time.
IMO, calving on green grass beats calving in a snowbank any day of the year. :banana: :banana: :banana: