According to this article, early JATO experiments used 18 rockets. Later versions used 33.

Although the B-47 was equipped with six 7200-pound-thrust (with water injection) turbojet engines, the thrust-to-weight ratio at maximum gross weight was only 0.22, which, coupled with its high stalling speed, resulted in a long takeoff ground roll. To reduce the takeoff field length, the aircraft was initially equipped with 18 short-duration booster rockets. These units were an integral part of the aircraft and were known by the acronym JATO (jet-assisted takeoff). Nine JATO nozzles were located on each side of the fuselage. On some later versions of the aircraft, weight was saved by replacing the integral JATO units with a jettisonable external rack containing the rockets. In this installation, 33 rockets of 1000 pounds thrust each were provided.

The B-47 was underpowered but it was a hell of a plane in its day. Here's a picture of a B-47 on a LABS (Low Altitude Bombing System) run. This bomb toss tactic was developed as a means of bombing from low altitude. It's basically an Immelman turn (half loop followed by a half roll).

Here's link to the Air Force Museum with info on every B-47 model. Just follow the links.