It depends on many factors, including the size of your country’s registry, your age and your sex.
For instance, in the UK, there are 550,000 registered donors. All donors on this list have a 1-in-900 chance of donating in the next 5 years. But if we focus on stronger, younger donors and just look at men aged 16-30, their chances go up to 1 in 200.
The odds aren’t super high, but they sure are higher than winning the lottery!