On a duplex (2 family home) how many outside outlet to you have to have??

Depends on they type of duplex. If the apartments are side by side, then 4 receptacle outlets. I each in front and back.

If the apartments are stacked, then 2 at the ground level and one in the fron and back of the upper apartment if front porch and / or rear deck are involved.

Depends on where you live... I have a two story single family home with a very large deck... I have ZERO outside outlets.

By code (at least 2008 through 2014 editions, possibly earlier), there are supposed to be a GFCI protected outlet receptacles in the front and rear.

In my case the property predates that period significantly... so its important to know when her building was constructed... and where.

My question is why are you asking? There has to be some reason for asking this.