I believe numeral 1 is omitted in Amadeus, so a quick bit of (very likely wrong) number crunching gives very roughly 1 in 5 of all letters, against 1 in 6000 of being all numbers. That assumes a random distribution which it may not be.

I only have 5 bookings in my account, but pleasingly one is all letters, which gives me some hope my calculations aren't wildly off.