Several. The plane operates as the first stage, basically. Only it's a far safer and more flexible first stage. Planes are well understood and completely reusab

Several. The plane operates as the first stage, basically. Only it's a far safer and more flexible first stage. Planes are well understood and completely reusable. The main limiting factor would be their carrying capacity. However, the limited carrying capacity of a supersonic aircraft is partially made up for by the reduction in launch weight the rocket would gain by being launched from several miles above sea level. But more importantly, a plane can take off from anywhere in the world and launch the rocket at any angle. There is an optimal orbital angle that can be taken from fixed ground-launch points, and moving off of that angle or achieving retrograde orbits requires significantly more fuel and thus reduces launch capacity significantly (see the challenges of reconciling the orbital angles achievable from both Cape Canaveral and Baikonour that had to be addressed in order to build the ISS). Plane-assisted launches do not require a fixed ground launch facility and can be launched in far more varied orbits. Point being, if we can make our satellites small enough, or launch them in self-assembling pieces, then we can achieve far more flexibility in orbital angles by using plane-assisted lauches.