How to generate card number so the users cannot follow how much is sold?

How to generate card number so the users cannot follow how much is sold?

I want some generator script to generate unique numbers but not in one
order. We need to sell tickets.
For example currently ticket numbers are like this:
100000
100001
100002
...
So the users can see how many are sold.
How can I generate unique numbers?
for example:
151647
457561
752163
...
I could use random number generator, but then I have always check in
database if such number has not been generated.
Hmm, maybe when using index on that column - the check would not take long.
Still now I have to get last card number, if I want to add 1 to it, but
getting last is fast enough.
And the more tickets will be sold, then bigger chance that RNG will
generate existing number. So migth be more checks in future. SO the best
would be to take last number and generate next by it.