Learn French: Le samedi, vous regardez un match. - On Saturdays, you watch a match.

In French, placing the word le before a day of the week shows that the action is a recurring habit. It means Every Saturday or On Saturdays. Notice that French does not add an s to samedi to make it plural here!

Is there a missing word for on in the French sentence?

No, French never uses a word for on before days of the week. For a single event you just say the day, and for a regular habit you only need to add le in front of it.

How do I know if vous means one person or a group of people in this sentence?

Without more context, it could be either! The word vous is used for a group of people, but it is also the formal way to address a single person. The verb regardez is spelled the same way in both situations.

Is match borrowed from English?

Yes, French uses the English word match to talk about a sports game. It is treated as a masculine noun, which is why it is paired with un.