How are there only 8 structural isomers of C5H11Br?
You'll probably need to draw all the possible isomers to see it, but with only 5 carbons to work with the ethyl isomers are redundant with the methyl isomers: i.e. 1-bromo-2-ethyl-propane is the same thing as 1-bromo-2-methyl-butane.

Remember we are only talking about structural isomers, and ignoring stereoisomers.