You can work as many hours as you like, so long as it does not affect your studies.
A sensible guideline for all full-time students is 12–15 hours per week. Remember that your studies need to take priority.
If you are an international student from outside the EU, you should check your visa.
You visa should tell you if you can work and for how many hours a week. If you are allowed to work, this will usually be for no more than 20 hours per week.
If you are unsure check with the Home Office or UKCISA.