It all depends on what kind of race you want to do, how competitive you want to be in that race, and what your strengths/weaknesses/level of fitness is. Are you just trying to finish, or are you trying to finish strong?

I know people that have completed sprint/olympic distance tri's without ever doing a brick workout while training. I also know people who do 2 or 3 brick workouts a week. Personally, I do one bike/run workout each week that would be considered a "brick". I might bike and run on one other day, but I will rest in between the two workouts (i.e. do one in the am, one in the pm).

It's really dependent on your training schedule. There isn't a rule of thumb that I'm aware of that says you need to do X bricks a week to be successful in triathlon races.