How many people are learning Chinese as a second language today?
Estimated by Ethnologue: 198.7 million as of 2021 (Mandarin Chinese only).
Note:

The data above is the total number of people who speak Mandarin Chinese (ISO 639-3: cmn) as their second language; it doesn't show differences between &quot;learning Chinese&quot; or &quot;speaking Chinese&quot;.
If we define Chinese language as a macrolanguage (ISO 639-3: zho), the number is beyond my knowledge.
Using Chinese as second language doesn't necessarily mean he/she is not of Chinese nationality. There are millions of minorities in China who speak their native language (Uyghur, Tibetan, Yi, Korean, etc. ) but use Mandarin Chinese as second language.
People who learn Chinese outside China are estimated to be 25 million according to MoE PRC (although this may be an exaggerated number, and may also include ethnic Chinese in Malaysia and Singapore).