?... than a thousand children have swimming lessons there every month

To provide a detailed explanation for the given incomplete sentence, we need to complete it with the comparative form of the missing word. Let's consider several options to complete the sentence based on comparative forms:

1. "...than a thousand children have swimming lessons there every month."
2. "...than a thousand children I saw have swimming lessons there every month."
3. "...than a thousand children in the other school have swimming lessons there every month."

The completion of the sentence depends on the context and the specific comparison being made.