Though I am still new to MythTV and LinuxMCE myself, I think I can answer your question.
You need as many tuners as you do the number of TV shows you want to record and watch simultaneously at a given moment. Meaning that if you have four tuners, you can: watch 1 show on the Living Room TV, watch 1 show in the bedroom and record two shows, OR watch 3 shows in the bedrooms and record 1 show, etc. So the number of tuners is based on the maximum TV usage at a given point. If you want to be able to watch TV in all rooms at the same time, I would also throw in an extra tuner or two to record shows on top of the four.
Though in order to assist you on which tuners to buy, I need to ask if you are using solely an antenna or do you have cable or cable/satellite boxes?