Not really care for, but the number of relationships you can keep track of:

"By using the average human brain size and extrapolating from the results of primates, he proposed that humans can comfortably maintain 150 stable relationships."

https://en.wikipedia.org/wiki/Dunbar%27s_number

There are actual close friends, and then there are people you know or care about to some degree. Dunbar's number (somewhere between 100 and 250) describes the latter.

That may include extended family, neighbors, coworkers, your acquaintances from hobbies/bar/church/whatever. They're not all close friends or immediate family, but still people that you sometimes interact with, and might want to contact.

I wouldn't say that the figure of Facebook friends is an indication of number of people I really cared. I do care some of them (sometimes sending wishes for them), but a majority of them are either my classmate in high school, a junior in university, a (former) co-worker, or someone whom I met in social gathering that shared common interests and wanted to hear/talk with them. It's like the use of Facebook as a directory since everyone has one.

Conversely, there are plenty of friends whom I know personally, but did not connect via Facebook (we use IM for that). This may probably be counted towards the close friends I really cared for; only few (no more than 10) people fits this criteria of mine.