I see people posted stuff on facebook telling their friends and family to stay safe. My question is shouldnt it be friends and families. Not friends and family.? or are they indicating one family as of their own family? what if the person is married and has kids? wouldnt that be 2 families?
A family is a group noun and in the expression "friends and family" the speaker is referring to their own family (meaning the people related to them). Depending on exact context or intent, that could be only people more closely related to them or may include more distant relations.
"Family" can occur in the plural, but that would be when you are referring to more than one group of people where each group consists of members related to each other.
My three brothers and I are taking our families on a trip this summer
In this case, each brother has a family (usually wife and kids).
As @P. E. Dant commented, "friends and family" has become idiomatic, largely (in my view) to the advertising of cellphone carrier service plans. So it is natural to say, whether grammatical or not.
Otherwise, your question could have two meanings:
1) friends and family - your friends and your family
In this case there is only one family (yours) so "family" singular is correct.
2) friends and (their) families - your friends and their families.
Now "families" plural is OK, but it doesn't make a lot of sense. Because:
1) Your family is not included
2) It would likely be an indeterminate number of people. And if you leave out "their", it could include all families in the world.
is used to describe a group of people who are closely related, usually genetically.
The singular also has the underlying meaning of having the speaker's affection
I consider them family.
My family includes my uncles, aunties and cousins.
When used this way, "family" can mean several generations
a family gathering
and a "family gathering" may consist of several individual families.
Each of my cousins brought their families to the family gathering.
"Families" (plural) here means the individual units whereas "family" (singular) refers to the overarching connection of everyone.