Is the word patience plural or singular?

Good question!

Patient can be a noun or an adjective. If we are speaking about the noun, then patients would be the plural of patient.

Since adjectives do not have a plural form in English, then patient as an adjective would be patient even if the noun it modifies is plural.

-They are very patient when it comes to standing in line at a bank in Costa Rica

Patience is also a noun. People must have a lot of patience when they travel to a different country.