Max cabin alt in an unpressurised a/c at which humans can continue to breathe normally............................14000'
Max cabin alt that you can function normally for a prolonged period...is approx..........................................10 000'
Now most pressurised a/c's use a Max cab alt of.....8000'
Since the Max cab alt is limited the next consideration is how much diff pressure can the fuselage take.....say about 8psi...
With these two figures in hand you can set about to figure out how high can you go.

Now another consideration is the capability(in term of time) of the a/c to reach say 14000' from its max alt