The object of the "I->you" form -lak/-lek may be either singular or plural. This sentence could mean "I love you guys." If you feel that the meaning is not clear enough from context, you can include the object: szeretlek téged for singular "you" and szeretlek titeket for plural.
Once I learned, that for the case of "I => singular you" = "Én => téged" can be expressed just by the verb with the suffix -lak / -lek.
Thats why: Szeretlek. => I love you.
For the case of "I => plural you" = "Én => titeket" you are not allowed to leave the accusative pronoun.
So: Szeretlek titeket. => I love you (guys).