There is no single optimal number — traditionally, scales with five to nine categories are used.

In general, the scale should be balanced in order to obtain objective results.

If a neutral or indifferent response is a viable option for some respondents, an odd number of categories should be chosen.

If some respondents may have no opinion, non-forced questions improve the accuracy of the results.

Not every point needs to be labeled — the key is to avoid ambiguity and to clearly name both poles (and, with an odd number, the midpoint as well).