I don't know of any planet with exactly 21 known moons.
The best answer is probably "Uranus" with 27 moons (natural satellites).
Neptune
Saturn is the only planet that has 3 rings and 16 satellites .....
Jupiter, with 63 satellites.
Jupiter
Satellites, can either be natural satellites (for example; Earth's moon), or man made satellites (for example; communication satellites).
Jupiter, Saturn and Uranus have MORE that 21 satellites.No planet has just 21 satellites - unless your are reading from a very old text book.
Neptune
uranus is the planet that have 27 satellites
You probably mean to ask about natural satellites as opposed to artificial satellites. In terms of neutrality, all satellites are neutral. In any event, the planet Jupiter has the most natural satellites, of any planet in our solar system. The planet Earth has the most artificial satellites.
Eclipse
There are no known satellites of Mercury.
Saturn is the only planet that has 3 rings and 16 satellites .....
Mercury & Venus have no natural satellites.
Venus has no known natural satellites (moons).
Jupiter, with 63 satellites.
No natural satellites of Mercury have been discovered.
The planet Mercury has no natural satellites.