Here's a top ten for native speakers. But of course languages such as English are used/understood by many non-native speakers:

Chinese* (937,132,000)
Spanish (332,000,000)
English (322,000,000)
Bengali (189,000,000)
Hindi/Urdu (182,000,000)
Arabic* (174,950,000)
Portuguese (170,000,000)
Russian (170,000,000)
Japanese (125,000,000)
German (98,000,000)
French* (79,572,000)

* The totals given for Chinese, Arabic, and French include more than one SIL variety.
This site, from which I got that, brings lots of information about such matters together.