A simple sentence may have one or more subjects (nouns/pronouns) and/or one or more predicates (verbs), but it has only one complete idea.

It has one main clause which in itself can stand as a sentence.

Read these sentences:

• My classmates and my teachers enjoy Harry's jokes.

• Civic leaders organized a prayer rally.

• My teachers, classmates, and schoolmates campaign for, promote, and observe environment preservation.

The number of thoughts or ideas is important to decide whether the sentence is a simple sentence or not. The number of subjects or predicates does not count.