A control flow path/control flow graph is a graphical representation of all paths that might be traversed through a program during its execution.
Now each $if$ statement has 2 outcomes ~ either true or false.
As per the grammar each if statement is independent of the other.
Consider 3 if statements
if e1 then e2 $(T)$ else e3 $(F)$;
if e4 then e5 $(T)$ else e6 $(F)$;
if e7 then e8 $(T)$ else e9 $(F)$;
Now following are the paths the program goes through during its execution :
$1. \ TTT$
$2. \ TTF$
$3. \ TFT$
$4. \ TFF$
$5. \ FTT$
$6. \ FTF$
$7. \ FFT$
$8. \ FFF$
So for $n = 3$ we have $2^{3} = 8$ control flow paths, hence for $n = 10$ we will have total $2^{10} = 1024$ control flow paths.