I don't have a cell.. But I will go ahead and substitute with my house phone.. If someone calls and I don't know the number, I usually let the machine get it, or I will answer it.. But usually even if I recognize the number I still don't answer it...
I have about 30, which are people I speak to on a regular or semi-regular basis. Anyone that I don't talk to often doesn't go in my phone. Not changed SIMs in about 3 years, so I'm probably due for a bit of a sort out.
Most of my contacts are close friends and family. As well as college, doctors etc. All the important people. Any emergency numbers have IOE: "In case of emergency" on the front of the names. I do have some unimportant people's numbers in my phone too. Just so I know who to ignore in case they try to call or txt. Once they stop, I delete the numbers. I never answer unidentified numbers.