I dont think there is any limit to the number of times, as they make $ out of each upgrade, BUT the price of that upgrade depends on which model you trade in.
so - if they announce a new model, lets say a 'P60', and you are upgrading from the current top model the P45+, there will be a price of $x. But if you are upgrading from an old P20 back, it would be much more expensive, $xxx.
it is not a limited number of upgrades, but the upgrade price changes.
anyway, for the best answer, ask a dealer.