In this case, it seems to me that your confusion is around whether "you" is singular or plural in the question. When you say "without knowing the outcome", I assume you're thinking that there might be a difference in the verb you use if there is only one person that "knows English" or many people that "know English".
The short answer is that the result of the question doesn't matter, we always use plural nouns with "many".
Many is used to describe count nouns or nouns that can be counted like books, ideas, leaves, and shoes. When using many, the noun will always be plural.
-- Merriam-Webster Learner's Dictionary
So in your question, the plural verb is required, and it would be:
How many of you know English?
If you wanted to use the third person singular verb, you might say:
Who here knows English?
Whether you may want to use "speaks English" instead of "knows English" would depend on the context. If you were traveling and wanted to find someone that you could talk to in English, you would prefer "speaks". For example "Does anyone here speak English?"
If you were asking a group of students to try to determine what level of English you would like to start teaching at, you might say "How many of you know some English already?" Those students may not be able to talk with you in English, but they might understand if you speak in English or be able to read English texts.