From the NS&I website:
"Odds
We hold a prize draw every month. There is a £1 million jackpot and, at the current odds, over 1 million other cash prizes. The winning Bond numbers are selected at random, and each £1 Bond number has a separate and equal chance of winning a prize.
The odds of winning are shown below. We calculate the number of prizes in each draw by dividing the total number of eligible Bond units by the odds: 24,000 to 1"
Each bond may have a separate and equal chance of being drawn but, clearly, the more you have the greater the chance of winning any single prize. If you hold the maximum (£30k from memory) and given the odds above then, with average luck, you should win 15 prizes per year.