hi xini, best is to take a Lee filter sampler at the shop you are buying gels.
Numbers are ok, but in the sampler you will have waves lengths drawn. This will help to choose something very near the wave length of anaglyphic glasses
Normally samplers are given, not solded.
There is 2 edition of samplers, numeric and artistic. Prefer to have an artistic one, where gels are sorted by color ( more easy)