Is there in Czech only one meaning of the word "number"? Because in Russian there are two words - "номер" (nomer, e.g. phone number, number of the room etc.) and "число" (chislo, e.g. the number "3", Fibonacci numbers etc.).
In Czech číslo has both these meanings.
Fibonacci's number and phone number are both číslo in Czech. Though typically asking a girl for a 'number' without specifying you do not mean Fibonacci's would do. :D