I don't think it would be prudent to measure the severity by the number of episodes per year. Episodes vary in length with the rate of cycling, so Person A could have 1 manic episode that lasts 1 month and depression lasting the other 11. Person B could be a rapid cycler, having 5 manic episodes a year, about a week long each, and spending the rest of the time in depression.

Both people spend about the same amount of time in mania, and about the same amount of time in depression.

A better measure of severity would be how highs the highs are, and how low the lows are. The higher the range between the upper and lower limits, that's how severe it is.

Another measure could be the intensity of the symptoms.