In English the word "knowledge" is always singular. For instance, you can't say someone has "a lot of knowledges" about something. It must be "a lot of knowledge."
In Russian, I have seen "level of knowledge" as both уровень знания and уровень знаний on various websites. What are some reasons, other than purely grammatical ones, to use one choice instead of the other? I am not interested in purely grammatical reasons such as много знаний, where много requires the plural form of the noun it modifies.