What are the common complications of diverticulitis, and how often do people need to have surgery for it?

Multiple. Perforation, obstruction, abscess and fistula formation. On occasion bleeding and stricture formation can complicate it. If a patient keeps having recurrent attacks requiring IV antibiotics and hospitalization, than surgery should be considered as an option to resolve the problem.
Lots & not often. There are several complications. If i list them, you'll look for them. Just rest assured, that for the most part, most patients are treated successfully with simply antibiotic therapy. If you fall into the minority, you'll cross that bridge when you get there. But don't worry, you don't have to look for the complications, they will find you - they are very apparent.