Why are there only seven continents?

 It's humans who make the lines, not the earth.

Thick plates drifting over deep fire.

Europe. Asia. They share one crust - it is unbroken.

If we call it Eurasia, there are six left over. If we go further south, we find a huge ocean shelf that has sunk long ago. If we call that one Zealandia, that makes eight.

Our maps lie to us. We split land by culture, but it makes hard borders with no water.

Hard rocks do not care that we name them. Large pieces slowly move away from each other. Ancient oceans dry up. New lands completely covered up.

The mapmakers. They find comfort in the numbers. Underneath the surface, the earth constantly moves, shifting under the heavy foot. Soon, the ice will melt, raising the black water to swallow the shores we know.

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It breaks through the boundaries we created long ago - Our beautiful drawing breaks apart yet again.

Did we know the shape of the continents before satellites?

 Here’s a 1950 map of Earth, 7 years before Sputnik launched:

Here’s a 1900AD map of Earth, 57 years before Sputnik:

And a 1900 globe projection of Earth:

And an 1850 map of Earth. It’s getting a bit rougher at the north and south poles, which were barely explored at the time.

An 1852 map:

A 1798 map of the world: not much worse than 1850.

By 1750, there were some real blanks at the far edges of the world from Europe:

1700 map - not much worse than 1750, allowing for the limits of the projection.

1630 map - missing a certain land down under:

1570: definitely some room for improvement, but the cartographer’s heart (and continents) are in the right places.

1502: a few holes here and there, but it was up to date with European explorations.

Short answer: yes, we figured out the shape of continents before satellites.