If human proteins are synthesized from only 20 different amino acids, then how can there be such variety of proteins in human cells?
There are only 26 letters in the English language, and more than 80% of words are under 10 letters. Yet there are over a million English words. Nonsense words don't count.

Now imagine the possibilities of a 20 letter alphabet, where the average "word" length is about 375 letters, and where words of up to 800 letters are possible. And that also does not include "nonsense words".

That should give you an idea of the possibilities, and why "thousands of different proteins in human cells" can exist.

How big is the “average” protein?