How many native speakers of Arabic are there?

It depends on how you define "Arabic". If you are referring exclusively to Modern Standard Arabic (Fus-ha), the official Arabic language, there are probably fewer than 10 million native speakers. Most people who speak MSA have learnt it as a second language. If you define "Arabic" as any of the Arabic dialects (some of which are not mutually intelligible), then you have roughly 290-300 million native speakers of Arabic.