In Sanskrit, it is एकविंश ekaviMza - but in Hindi it becomes ekkis.
It looks like some uneducated person mistook "विं" viN (not M) as Ki and made it ekkis.
And this carried to Greek to become eikosi but meant 20. This can happen if Indians were counting from 1 and Greeks from 0. Or Greeks were counting steps between numbers.
We know Decimal System developed in India and was introduced to Europe by Fibonacci.
It is one proof that Sanskrit developed before Ancient Greek and Greek was one of the Colonies like other States of India and European Countries. This is also confirmed by all PIE DNA, Archaeology and Linguistics Studies.