Up to 10, named $CUSTOM1$ through $CUSTOM10$.

In the file they occupy Column C to Column L. Column A is Phone and Column B is Message when you are putting the message in the file.

If you need more than ten pieces of personalised data in one message, combine them in the source system before export. For example, join a first and last name into one column instead of using two variables.

See How to Send Customised SMS Using File Upload for the full structure.