One solution is to repeat/use multiple spectators. I would also not contradict the spectators when they ask...remember the key to improv: "yes, and..."
Also, be clear about what the theatrical mechanism of the effect is. Are you seeing the future? Is your understanding of human nature so deep that you are able to accurately know the factors that go into the decision of a "random" number...If the latter is the case, there is nothing wrong with going for an "honest" answer and giving them the reasons you put down the number you did.
I would also look at the relative strength of the rest of your repertoire. If you are doing work that reads as "charming, entertaining tricks" and, you throw in something that feels like direct mindreading, the context of the rest of your act points the audience to a more banal solution. If you hit them with mind-altering miracle after mind-altering miracle -- they are much less likely to suppose that "everyone just picks the same number..."
But, again, if you're getting this question; the key to turning it into an entertaining "moment" is not to contradict the audience. Have the complete inner conviction that everyone always picks 22. "I don't know why, it just works every time. Think of a number..." as you begin to repeat the effect...there should be some comic tension when you've just said 'they think of it every time,' and you ask someone to think of a number. Hit it, riff on the result...maybe hit it again...just feel out the audience and have fun with them...
If you're feeling particularly wicked, rather than repeating, you might say "I don't know why, it just works every time..try it yourself next time you want to impress someone."