Five points almost always, seven if you have volume
Five points are the compromise that works: enough to discriminate, few enough to read at a glance on a phone. Seven give more resolution, useful when comparing groups or spotting small shifts between two rounds, but you need enough responses for that resolution not to be noise.
Three points («bad, so-so, good») are legitimate and often underrated: if the question only needs to sort people, three options are answered faster and produce less hesitation. Ten points, on the other hand, are nearly always a mistake: nobody genuinely distinguishes 6 from 7, and the difference you see in the data is chance.
The midpoint: include it or not
An odd number of points has a neutral middle; an even number forces a side. Removing the midpoint artificially raises the share of «positive» answers, and that's worth knowing before comparing two questionnaires built differently.
The rule I use: keep the midpoint when having no opinion is plausible, and drop it only when the question is about something the respondent has certainly experienced. Removing it «to force a decision» just shifts answers towards the positive side.
Label the ends, not every point
Writing «Not at all» under 1 and «Very much» under 5 is enough and it reads. Labelling all five points («somewhat agree», «rather agree») introduces linguistic shades that everyone interprets differently, and on a numeric scale it ends up making answers less comparable, not more.
What always needs stating is the direction: is 1 the low end or the high one? It's obvious to whoever wrote the question and it isn't to whoever reads it. End labels solve exactly that.
When a matrix beats five questions
If you're about to ask four consecutive questions on the same scale («the teacher was clear», «the materials were useful»…), that's a matrix: statements as rows, the scale as columns. One screen instead of four, same data, and the respondent sees straight away that it's a single block to get through.
The limit is width: more than five columns on a phone becomes unreadable. If you need seven points, go back to separate questions.
Five points, keep the midpoint unless you have a reason not to, label only the ends — and if consecutive questions share a scale, use a matrix.