I think the absolute maximum luck you can have on your first real round is around 120 strokes

First of all, my personal best is 91 and I'm usually a "white stone" (beginner), so if you disagree, I'm probably wrong.
This is based on a golfer who basically has a proper swing.
If a screen golfer who shoots 100 went out for their first real round, they'd hit 144 double pars (yanga) if they just played by the rules without looking.
Now... let's say I have a decent swing? Even then, bunker approaches are impossible on the first day.
Even if I'm being very generous, one-third of the 18 holes will inevitably be double pars. So, roughly 72/3 x 2, we start with 48.
On the lucky holes, you might hit a driver into a hazard on a Par 4... but there's a mat with a hazard tee in a good spot, you hit a shot of a lifetime, and land on the green.
You'll miss putts, but somehow (suspiciously long) get a gimme, a two-putt, or a three-putt for a bogey or double bogey.
For the rest, if you mix in some Pars, Double Pars, Par Fours, Triple Pars, Par Fives, Triple Pars, and Quadruple Pars, you'll end up around 120 strokes.
If you went out for your first round and shot under 120? You were playing with some local "neighborhood rules" that have no pedigree lol