In calling Bekannt- substantiviertes Adjektiv you basically answered your own question. The correct ending can be determined by looking at a table of adjective inflection. However, I still want to try to get to the bottom of the problem.
If I understand your question correctly, the ending -n is giving you problems. Remember that this -n belongs to the declension of adjectives, not nouns! Coming from nouns, you might associate -n with the plural and the plural only. However, this is different for adjectives.
Looking first at the plural with the seemingly missing -n, we simply have the strong adjective ending -e:
Annas beste Freundin (singular), Annas beste Freundinnen (plural)
Annas Bekannte (singular or plural)
To get the weak ending -en in the plural, substitute the genitive Annas by an article with an ending:
Annas beste Freundinnen > ihre besten Freundinnen
Annas Bekannte > ihre Bekannten (plural only)
Regarding the seemingly unexpected -n in the singular, it is the weak adjective ending -en:
ihrer besten Freudin (dative or genitive)