A number of sources have compiled lists of languages by their number of speakers. However, all such lists should be used with caution.
Ethnologue (2015, 18th edition)
The following languages are listed as having 50 million or more native speakers in the 2015 edition of Ethnologue, a language reference published by SIL International based in the US. Speaker totals are generally not reliable, as they add together estimates from different dates and (usually uncited) sources; language information is not collected on most national censuses.
Hausa has 25 million L1 total and 15 million L2 in Nigeria, and so approaches our limit of 50 million. Coastal Swahili has 15 million L1 in Tanzania (2012) and "probably over 80% of rural" Tanzania as L2, not counting Kenya or the 10 million L2 speakers of Congo Swahili (1999), so it also approaches our limit.