What is the best way to express "relative(s)" in Russian?
For example...
1) You're my favorite relative.
2) How many relatives do you have?
I've come across some candidates (e.g., родные, родня, родственник) but don't know what the differences between them are in meaning and/or usage.
Родня and родные are collective nouns.
Родные and родня are close in meaning, however родные more often than not means "close family", those who you maintain relationship with, while родня usually means "extended family", those who are related to you, even if you are not in contact with them.
If you are interested if a person comes from a numerous family, you'd ask:
У тебя много родни?
If you want to ask if a person has a relative they can rely on, you ask:
У тебя есть кто-нибудь из родных?
If you are interested in exact number of relatives, you can't use collective nouns, so you'd just ask:
Сколько у тебя родственников?
Родня in broad meaning is the whole bunch of your relatives, kinsfolk. In narrow meaning родня can be used for one or several persons - "Он мне родня (he's my relative)", "Они мне родня (they are my relatives)".
Родные is mainly the same as родня. Note that "родной" has also a meaning of genetic relation (as in "родной брат" (not step-brother).
Родственник is one of your relatives, one person.
As for your examples:
The most natural here is "родственник" - Ты мой самый любимый родственник. "Ты моя самая любимая родня" is also possible but would be very colloquial.
Here you can say "сколько у тебя родственников?" or "сколько у тебя родных?" But the second sounds unnatural.
In addition, родня and родные are familiar words while родственник(и) is much formal and widespread word.
