According to the Encyclopedia Britannica, there are 143 species of bovids. The Animal Diversity Web is a bit less precise, indicating that there are "more than 140 extant and 300 extinct species." That second number is highly questionable because today there exist sister species that are so similar I doubt they could be told apart from fossils alone. If you check around the internet, this ~140 number comes up again and again, and Wikipedia says 143.