I don't actually NEED any. My current job doesn't require me to be armed, and my lifestyle is such that the chances of me being a victim of a violent crime where my only option for survival is armed resistance are very small.
That being said I do choose to occasionally carry and have a handgun for home defense. My number boils down to three:
1. Pocket gun for carry anytime, in any clothing choice.
2. Larger gun for carry when wearing a concealment garment or when concealment is not important.
3. Something for fun on the range (depends on your idea of fun).
These picks assume that you feel you need to carry something, and that you enjoy going to the range.
I think the minimum set will be different for all shooters, depending on their particular interests as a hobbyist or sportsperson. If it's JUST for SD, then two should cover it. Pocket and non-pocket.
If not interested in SD, then one of each type that you use in your chosen sport / competition / aspect of the hobby.