You have 3 options that you can choose from for this use case:
Use our 'Generate Random Numbers for Records' automation. It allows you to generate alphanumeric numbers as well.
Use an Autonumber field
Use the Record ID
Using the Record ID will be the best solution if you require unique values, since every record will always have a unique ID, whereas the other two options are not guaranteed to be always unique, depending on your exact use case.