If the game modeled everything historically:
You bring 11 ships, led by an explorer named Cortes to Central America. You park your boats in a native province despite not getting fleet rights, and then the crew from your 11 ships magically form into a 500 men regiment, leaving the ships empty.
Oh, and somehow, 15 horses and 15 cannons from the ship were brought along, and I'm not sure how the game can model this - maybe a new unit type that has some sort of combined arms bonus, and 15 cavalry is useless, and 15 cannons equals 1/3 of a regiment, but without any people to man it....
Anyway, your explorer Cortes magically turns into a conquistador named Cortes, and now leads your army which in EU IV terms is less than a single regiment (about half!).
You then go on and recruit from enemy territory, about 2-3 units of infantry mercenaries, boosting your force to about 3.5 regiments. Then you win a series of battles against various stacks of 20-40k units, take the king hostage (totally a button for this), and hire another 100k of local mercenaries (or make allies during war, whichever is more possible in EU IV). Finally defeating a doom stack of 300k of Aztec and their allies, taking the capital and annex all of Aztec lands.
Sounds about right!
/lol
But really, the game can't model that almost absurd (but real) history, so instead Castile ships 20k stacks and crushes the natives and takes all the land. Same result, just modeled in way the game can actually understand.
So I think, as long as results are modeled well, the exact meaning of 300 Heavy ships in the later eras can be seen as just a model, as long as all the country's varying navy sizes are to scale. I mean, if they game went all the way to today, maybe the USA has 1300 Heavy Ships...and all that really means is 13 aircraft carriers.