You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Estonia

Context:
Modern Estonia is a fairly ethnically heterogeneous country, but this heterogeneity is not a feature of much of the country as the non-Estonian population is concentrated in two of Estonia's counties. Thirteen of Estonia's 15 counties are over 80% ethnic Estonian, the most homogeneous being Hiiumaa, where Estonians account for 98.4% of the population. In the counties of Harju (including the capital city, Tallinn) and Ida-Viru, however, ethnic Estonians make up 60% and 20% of the population, respectively. Russians make up 25.6% of the total population but account for 36% of the population in Harju county and 70% of the population in Ida-Viru county.

Question:
How many counties are in Estonia?

15