How do you create a for loop with a changeable number of iterations?

You can use seq

for i in $(seq 1 "$num") 

or your shell may support C-style loops e.g. in bash

for ((i=0; i<$num; i++))

echo "how many times would you like to run your loop ?"
read  max

i=0
while [ "$((i+=1))" -le "$max" ]
do
   # whatever you want to do here
done