Label expressions are similar to field calculator: you don't need to use a
for loop as the expression is already applied to every feature. Also, you'll get an error with your code as per the help:
Field values are automatically cast to text strings. Therefore, if you
wish to use a numeric value in an arithmetic operation, or when making
a comparison, you will need to cast it back to a numeric data type.
The examples below add two integer fields
Here it is displaying odd values:
return [Frame_Number] if int([Frame_Number]) % 2 else "whatever you want here"
Alternatively, you can use a SQL expression in Label Manager to accomplish the same:
MOD(Frame_Number, 2) = 1