When do I use these words.....

[Basic of the life]

Hellow,
I often read those words:
approximately....nearly....almost
But I don't know..When can I use them?

I'm waiting for your help.
Best regards,

[Tdol]

approximately 20 people- could be a few more or a few less
nearly/almost- these are both under the limit, so nearly twenty can only be fewer