Question

Once you understand exactly how to perform each resistance training exercise, you should choose a weight that allows you to do how many repetitions.

Answer 1

This mostl depends on your goals.

If you would like to gain muscle mass and incease your size, then a repetition range of 8 to 12 repetitions is good for you.
If you would like to increase your endurance then go for more than that.
If you would like to increase your strength then go for less than that.