You can have multiple I-140 filed but only one in each category per sponsor at the same time at a service center. So you can only have one self-petitioned NIW I-140, strictly speaking. However you can have others such as one self-petitioned in EB1a, one in EB1b by a sponsor, one in EB3 using LC by a sponsor, one in EB2a/EB3 using another LC,.. as long as you are eligible for those.