I voted two single, although if you think your kids will want to ride a lot, I'd bring a single for the infant and rent a double. Two strollers allows your group to split up if necessary. If the 6-month old is napping in the stroller, the other parent can take the other two kids on rides (including across the park, if necessary).
The Disney's strollers don't recline, so I'd bring one from home for the littlest one.
If your six month old doesn't sit up in restaurant high chairs yet, I'd bring the stroller and infant carrier so you can use the carrier in restaurants. If your baby can sit in the high chairs, I'd bring a lightweight but reclinable stroller for him/her.
I'd expect your 3/4 year old (you have both ages listed, so I'm guessing that child turns 4 before the trip) to want to be in a stroller some. DD didn't use a stroller at 5, but she did at 4. If you think your seven year old will want to catch a ride from time to time, I'd rent a double, even though they are expensive. I probably wouldn't lug a single and a double from home. Your 7 year old might want to ride in Epcot and maybe AK, but DHS is pretty small and MK is still very walkable for a 7 year old. When my kids were 7 and 4, we rented a double and used the second seat for the camera bag and backpack (they weren't so expensive to rent back then). DD7 would hop in if we were in a hurry to get somewhere or if she wanted a place to sit to wait for the parade to start, but she didn't have any trouble walking through the parks.