We shouldn't assume any port or re-creation of Galaga is faithful to the original but it is interesting to see how the original game handled this, at least as a starting point.
1. Namco Galaga 1981 Coin-operated Arcade machine
Galaga was created by Namco but in the USA it was licenced by Miday, they manufactured the cabinets sold in the USA and produced a
"Parts and Operating Manual" for Galaga:
On Page 2-5 it shows the operator's switch-settable options for extra lives:
So depending on how the operator set-up their machine, you would get an extra ship at 20,000 points another at 60,000 points and at each multiple of 60,000 points.
However the operator could set the machine so that you never get a bonus ship.
2. Namco Arcade Game Series: Galaga - available on Steam for PC.
This is an officially licenced port of Namco's Galaga.
In this game, as of 2020, there is a game-menu you can reach by pressing Esc, you can select
This offers exactly the same options as the original coin-operated arcade machine but defaults to the second set of options for extra lives (maybe this was also the default out of the factory for the original machine)
3. Galaga ROMs in MAME 0.217
If you press Tab while running Galaga in MAME you get a menu
If you select
DIP switches you get the same range of choices as in the original manual and in the Steam version from Namco.
Again, there are the same range of options for extra lives and the default is the 20,000 / 70,000 / 70,000 option.