> at this point I've had 4 callups.
Well, since we're doing random anecdotal evidence ... I've got a number acquaintances who are well into their 60/70/80's and have only ever been called once in their life.
I would suggest more than once is the exception rather than the rule.
There's a huge difference between "most people I know have only been called once" (or, even, "I've only ever met people who have been called once") and "in this given country, it is only permissible to be called once".
Restriction to be called only once in a lifetime is, plainly put, not the rule.
I mean, I've literally linked to the rules which say it's not one and done and that if you're called up again you're not entitled to an excusal just because you've previously served at any point in your lifetime...
But yes, I do also know people who have been called up at most once. That is the nature of random selection.