A letter that says a student is hardworking and capable says nothing, because every letter says that. A letter that describes a specific moment in a specific classroom says a great deal.
Who to ask
The teacher who can write concretely, not the one with the most senior position. A teacher who taught the student recently, in a subject related to the intended field, and who has seen the student do something beyond the required work.
A well known name attached to a generic letter is weaker than an unknown name attached to a specific one.
The usual structure
Most applications ask for two subject teachers and the school counsellor. Some ask for one teacher, some accept an additional letter from outside school.
Where two subject teachers are required, choosing them from different areas usually gives a fuller picture than two from the same department.
How to ask
In person where possible, well before the deadline, and with something to work from: which universities, which programmes, and a reminder of the specific work the student did in that class.
Teachers write many of these letters in the same weeks. A student who supplies the specifics makes a specific letter possible; a student who does not is relying on memory.
Not seeing the letter
Applications generally ask the student to waive the right to read the letter, and letters submitted under a waiver carry more weight, because a reader knows the letter was written candidly.
This means the choice of writer is the only control the student has, which is why it matters more than the request itself.
Adding more letters
An additional letter helps only when it adds something the others cannot, such as a research supervisor or a coach in a field central to the application.
More letters saying the same thing dilute rather than strengthen, and some institutions decline to read beyond the required number.
Following up
Letters are submitted by the teacher, and the platform shows whether they have arrived. Checking a few weeks before the deadline, and asking once more if nothing has appeared, is part of the student's job rather than an imposition.