Please answer a question about the following article about Sanskrit:

Many Sanskrit dramas also indicate that the language coexisted with Prakrits, spoken by multilingual speakers with a more extensive education. Sanskrit speakers were almost always multilingual. In the medieval era, Sanskrit continued to be spoken and written, particularly by learned Brahmins for scholarly communication. This was a thin layer of Indian society, but covered a wide geography. Centres like Varanasi, Paithan, Pune and Kanchipuram had a strong presence of teaching and debating institutions, and high classical Sanskrit was maintained until British times.

How large of a group in society use Sanskrit?
thin layer