How many questions should an exit ticket have?

Two or three. Enough to distinguish understood, nearly and not yet, short enough that the class finishes before the bell. An unfinished ticket tells you about your pacing rather than their understanding.

What is the difference between an exit ticket and a starter?

Timing and purpose, not format. An exit ticket at the end checks what was learned today; a starter at the beginning checks what survived from yesterday; a warm-up primes what today needs.

Should exit tickets be marked?

Read them, but do not award marks. They are formative - the point is to inform tomorrow's teaching, not to produce a grade. Marking them changes what pupils write, and you will get performances instead of honest answers.

What do I do with the results?

Sort into got it, nearly, and not yet, then decide whether to move on, reteach, or pull a small group. If most of the class is in not yet, that is feedback about the lesson.

Exit tickets: formative assessment that fits in five minutes

An exit ticket should ask two or three questions about the single thing you most need to know before teaching this class again, and it is only worth doing if you act on the pile the next morning. The questions are the easy part; the discipline is keeping it small enough to finish before the bell and actually using what it tells you.

Ask about one thing

The most common mistake is treating an exit ticket as a mini test covering the lesson. It is not a test - it is a check on the one idea that determines whether tomorrow's lesson can proceed as planned.

Before writing it, finish this sentence: "I cannot teach tomorrow's lesson unless they can ___." Whatever fills that blank is the exit ticket. Everything else is interesting and can wait.

Two or three questions, and why the format matters

Two or three is enough to distinguish "understood", "nearly" and "not yet" while still being finishable in the last minutes of a lesson. Longer tickets do not get finished, and an unfinished ticket tells you about pace rather than understanding.

The format should follow what you intend to do with the answers.

Multiple choice: When you want to sort a pile in thirty seconds tonight. Make the wrong options real misconceptions rather than obvious throwaways, and it tells you which mistake they made, not just that they made one.
Short answer: When you need to see reasoning. Slower to read, and the only format that shows you a pupil got the right answer for the wrong reason.
One of each: A common compromise: one to sort quickly, one to read properly.

The part most guidance skips: the next morning

An exit ticket that is collected and not read is worse than none, because it costs lesson time and teaches pupils that what they write is not looked at.

Sort the pile into three: got it, nearly, not yet. That takes a few minutes and immediately answers a question you would otherwise guess at - whether to move on, reteach to everyone, or pull a small group while the rest carry on. It is also the cheapest way to decide who needs which version of a differentiated worksheet tomorrow.

If the "not yet" pile is most of the class, the lesson needs reteaching and that is information about the lesson rather than the pupils.

Starters and warm-ups are the same tool with a different purpose

The same two or three questions asked at the start of a lesson do a different job: a starter checks what survived from yesterday, and a warm-up primes what is needed today.

Running one at the start and one at the end of the same lesson is a legitimately powerful pairing - the pair tells you what the lesson itself changed, which no single check can.

Questions

How many questions should an exit ticket have?: Two or three. Enough to distinguish understood, nearly and not yet, short enough that the class finishes before the bell. An unfinished ticket tells you about your pacing rather than their understanding.
What is the difference between an exit ticket and a starter?: Timing and purpose, not format. An exit ticket at the end checks what was learned today; a starter at the beginning checks what survived from yesterday; a warm-up primes what today needs.
Should exit tickets be marked?: Read them, but do not award marks. They are formative - the point is to inform tomorrow's teaching, not to produce a grade. Marking them changes what pupils write, and you will get performances instead of honest answers.
What do I do with the results?: Sort into got it, nearly, and not yet, then decide whether to move on, reteach, or pull a small group. If most of the class is in not yet, that is feedback about the lesson.