What are the chances of getting pregnant each month before ovulation does it always happen or is it only a 20% chance not a guarantee?

No guarantee in life: As Ben Franklin's saying goes, "In this world nothing can be said to be certain, except death and taxes." Life is about "likelihoods". If there is a 20% chance of pregnancy each month for a fertile couple having daily sex, it merely means that for 100 such couples, 20 will be pregnant by month's end. You cannot predict if you will be one of the 20 pregnant ones or one of the 80 non-pregnant ones.