Languages come in tiers. English is the global lingua franca. People use it to speak to anyone, no matter whether English native speaker or not. If someone from Norway wants to talk to someone from Japan, they'll most likely use English since both of them likely speak it.

Then there's regional lingua francas, languages like Spanish, Russian or Mandarin. These languages are popular in specific parts of the world and often used to get around there. Someone from Ukraine can speak to someone from Belarus using Russian.

Lastly, there's local languages that are spoken only in a country (or even only a part of a country). People speak them because that's what they were grown up with.

So in general, there's 4 "language slots" of languages people speak:

• The global lingua franca
• The regional lingua franca
• The language of the country they live in
• The language they grew up with

One language can fill multiple slots.

So for example, if you grew up in Ukraine and moved to Germany, you might speak the following languages, according to the slots above:

• English
• Russian
• German
• Ukranian

If you are born in Wales and never moved away, it might look like this:

• English
• English
• English
• Welsh

If you spent your life in the US, it would be like this:

• English
• English
• English
• English

This is the reason why people living in countries with lower-tier languages frequently speak 3-4 languages, while English native speakers really struggle to even learn the basics of one additional language. Because the former group has an actual use for more than one language, while the latter one don't.

Source? I think speaking one language is pretty rare. Most Europeans speak at least two, most Africans I've met speak 3, lots of Indians speak 3 as well...

I think anyone in India and Africa speaks 4 languages easily.

1. their regional language (i.e. Masaai, Yoruba, Xhosa)
2. the over-regional language (Arabic, Swahili)
3. a coloniser language (English, French)
4. and possibly just enough of a neighbouring regional language

I think many Chinese people are also bilingual (i.e. Wu+ always mandarin). They often learn another language in school (English or something geographically closer, like Korean).

Well, if you add up the number of speakers of second languages according to this page, and assume anybody speaks at least one language as their first one, you'll end up with almost exactly 1.4 as the average number of languages any given human speaks.
That's the lower bound, though, as I only added up second languages where the number of speakers is at least one million, and Wikipedia doesn't list many more anyway.