Here are some possible responses:
If you know the exact number:
* Hay [number] estudiantes en la clase de español. (There are [number] students in the Spanish class.)
* Example: Hay 25 estudiantes en la clase de español.
If you don't know the exact number:
* No lo sé, pero hay muchos/muchas estudiantes. (I don't know, but there are many students.)
* Hay bastantes estudiantes. (There are quite a few students.)
* No hay muchos/muchas estudiantes. (There aren't many students.)
You can also use a question to ask for the information:
* ¿Puedes decirme cuántos estudiantes hay en la clase de español? (Can you tell me how many students are in the Spanish class?)
