How do you pronounce the symbol $'$ in $f'$?
Solution 1:
It varies with the region.
"Prime" is how you pronounce it in American English. Here is a nice reference for American English pronounciations of math symbols; $f'$ is on the top of the second page.
"Dash" is how you pronounce it in British English: Here (search for dash).
Solution 2:
$f'\rightarrow$ f prime
$f^{\prime\prime}\rightarrow$ f bis
$f^{\prime\prime\prime}\rightarrow$ f tris
$f^{\prime\ \backprime\prime}\rightarrow$ f tetrakis
However most people do not know these words and just say double prime, triple prime etc.