"Mood" can be both a count noun and a non-count noun:
* Non-count noun: When referring to a general emotional state or atmosphere, "mood" is non-count. You wouldn't say "three moods," for example.
* Examples:
* "The mood in the room was tense."
* "He's in a bad mood today."
* "The music created a peaceful mood."
* Count noun: When referring to a specific, identifiable emotional state, "mood" can be a count noun.
* Examples:
* "She was in a contemplative mood."
* "He has a variety of moods."
In short: If you're talking about a general feeling or atmosphere, it's non-count. If you're talking about specific, distinct emotions, it's countable.