A number of sources have compiled lists of languages by their number of speakers. However, all such lists should be used with caution.
First, it is difficult to define exactly what constitutes a language as opposed to a dialect. For example, Chinese and Arabic are sometimes considered single languages and sometimes language families. Similarly, Hindi is sometimes considered a single language or a family including Mewari, Chattisgarhi, Bhojpuri etc., but together with Urdu it also is often considered a single language Hindustani.
Second, there is no single criterion for how much knowledge is sufficient to be counted as a second-language speaker. For example, English has about 350 million native speakers but, depending on the criterion chosen, can be said to have as many as 2 billion speakers.
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. Speaker totals are generally not reliable, as they add together estimates from different dates.
The distinction Ethnologue uses for Eastern and Western Panjabi is the national border, which does not correspond to the linguistic distinction. Hindi and Urdu are essentially a single language; however, 100 million non-Hindustani speakers are included under "Hindi", which is therefore not a single language. 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.