In 1999 Arabic was estimated to have about 206 native speakers (though by some other estimates the figure is much higher, nearer 280 million) and about 450 million speakers in total - this figure includes people who aren't fluent but have some knowledge of the language. This makes it the fifth most popular language in the world (the top language is of course Mandarin, which has over a billon speakers in total).
Some of the variation in figures can be explained by the different forms of the language. Like many languages, Arabic has a number of forms spoken in different countries, especially around the Middle East. Some people who speak one or other of these forms don't use the standard form, which is usually known as Modern Standard Arabic - the written form. So some calculations focus on the standard form, and others on total speakers of any variety.
It is also hard to estimate total speakers because a great many Muslims, or people interested in Islam, learn at least some Arabic through studying the Koran, but may not show up on figures as Arabic speakers.