"Nie mam pięciu krzeseł."

Translation: I do not have five chairs.

[Oba80]

is it also for numbers to be declined the same way as other nouns?

[Jellei]

Yes. I'm afraid that numerals are among the most difficult things in Polish.

[Oba80]

Also I think so, but it is interesting trying to learn it. Thank you very much for your great Help

[Pete357534]

She seems to be accenting the second syllable in krzeseł. Does it sound ok?

[jagonics]

It doesn't sound ok. In the word "krzeseł" you should stress slightly the first syllable.

[guille253]

What is the diference between pięć (five) and pięciu (five) ?

[Jellei]

"pięciu" is Genitive, as needed in this sentence. "mieć" takes Accusative, it's negated here, and negated Accusative results in Genitive.

"pięć" is the basic, Nominative form. And Accusative as well.