Of course, colloquially, Finns typically shorten them to

yy kah kol nel vii kuu sei kasi ysi kymppi

(or a similar variation)

with multiples of ten shorted to -kyt. So twenty-one is 'kahkytyy', the same number of syllables as in English.

Ref: http://en.wikipedia.org/wiki/Colloquial_Finnish#Numerals

Learning Finnish involves learning two dialects, spoken and written, and the spoken dialect can vary considerably even between cities 200km apart. But it's an incredibly fun language to learn. Very logically constructed, few(er) exceptions to the rules, and if you can say it, you can spell it!