How many solutions (velocities) exist in case of Lambert's problem?
Lambert's problem has an infinite number of solutions. For example, there's typically a solution that involves more that 0° but less than 180° on the transfer orbit, another than involves more than 180° but less than 360° on the transfer orbit, yet another than involves more than 360° but less than 540°, and so on. There are also solutions that involve reversing direction, essentially equivalent to less than 0° on the transfer orbit.
The long transfers (anything requiring more than 360° on the transfer orbit) take too much time, are very touchy, and are hard to find. They're not worth looking at. The solutions that require reversing direction are extremely expensive in terms of delta V. There's no point in looking at those solutions, either.
That leaves two solutions that are worthy of investigation, the "short way" (more than 0° but less than 180° on the transfer orbit) and the "long way" (more than 180° but less than 360° on the transfer orbit). You need to investigate both.