is knowledge in Arabic..
Arabic does not use infinitives. Verbforms are refered to under the perfect form of the 3rd person singular.
عرف - arafa
Maarifa is similar to "Mara ife" that is "know sth" in Igbo language
Is there any plural form????
No, (as in English) it follows class 6 (ji/ma-) agreement, and is a loanword from Arabic.
According to the English Wiktionari, maarifa is a plural-only word.