it's not about the number of defenders only that will make your defense strong,it's about the quality of your players too

for example I use 2 or 3 defenders against weaker opponents who play with 5 defenders and 1 DMC and usually win with 2 goals difference at least although my team doesnt have any scout players or 6* players

you will need to work on your defenders and train them regularly in addition to buy 1 or 2 quality defenders and push your players to press high and get the danger out of your defense

it works with me like this