er, no, let's be accurate here. you cannot expect to win 'most months'. the current 'specified number', which determines the number of prizes and hence the odds of winning a prize, is indeed 24000 exactly at the moment. so if you had over �24000 invested then in any given month, you are more likely to win a prize than not, so you could expect to win a prize 'most months' (more than 50% of months, on average). With �20k invested, you can expect to win a prize roughly 5 times per year on average (my probability isn't strong enough to confirm this exactly).
The current effective interest rate is 3.6% so you can expect to win �720 / year on average, tax free, assuming you do not reinvest your prizes. Of course there is no guarantee of winning anything, but you might win a million (or more!) in your first draw!