As you may have guessed by now, mostly cloudy means there are more clouds than sun (or stars, at night). The NWS definition says the sky is classified as mostly cloudy when 3/4 to 7/8 of the sky is covered by clouds, and it can also be referred to as "considerable cloudiness."

Which has more clouds partly sunny or partly cloudy?

The sky condition is based on the amount of clouds expected. There is no difference between partly cloudy and partly sunny. They both represent the percentage of sky cover. It is up to the forecaster how they want to represent the forecast each day.

What's the difference between mostly cloudy and cloudy?

“Partly cloudy” refers to cloud coverage from two through five tenths; “mostly cloudy” is used in reference to six through nine tenths cloud coverage; and “cloudy” to a complete or nearly complete overcast.