It depends on several factors, but here is what we recommend:
Use a dedicated IP:
You are sending more than 50k email per week, in order to isolate your reputation.
Use a shared IP:
If you are sending less than 5,000 messages a day, a shared IP may be the right solution for you.
Your email sending is volatile with large spikes of volume, may assume those large spikes are spam.
Use a pool of dedicated IPs:
You are a high volume sender, limit the total volume per IP per hour.
To separate IPs for your bulk and transactional mail.