how many hydrogen atoms are in acyclic alkane with 8 carbon atoms?

The general formula of acyclic alkane is CnH2n+2.

When n = 8 :
Number of hydrogen atoms = 2n + 2 = 18

In organic chemistry, an alkane, or paraffin (a historical name that also has other meanings), is a saturated hydrocarbon. Alkanes consist only of hydrogen and carbon atoms and all bonds are single bonds. Alkanes (technically, always acyclic or open-chain compounds) have the general chemical formula CnH2n+2.
here no of carbons is 8
so number of hydrogen = 2*8+2
=16+2=18

I have to assume that you mean a cyclic alkane.
In a cyclic alkane the general formula is: CnH2n
If C= 8 , then H = 16
cyclooctane = C8H16.

Or did you mean "A cyclic alkane"? Subtract H's.