"I know you."
Yes, OK. It is a little bit confusing, because two persons are involved.
ÉN ismerlek - that is always singular. It is I. First person singular.
The other person is whom the verb refers to. The object of the verb. That can be both the singular and plural informal you.
Én ismerlek téged. - I know you (one person).
Én ismerlek titeket. - I know you (a group of people).
The verb form is the same for the two.