How many days did you show up and do the work during your PhD?
How many papers did you read?
How many questions did you ask yourself or others?
How many times were you called on to respond to a question?
How many times did you present something?
How many times did you sit down at a keyboard to write your thesis?
How many people have told you something you did was good?
A few of these questions will have exact answers; others, might have ballpark responses you could guess. But all together, the totals tell you – and others – that you are ready for your viva when it comes. You have done the work, you have what it takes, you are ready.
And if you need to, you have time to increase those totals: keep showing up, keep sharing your work, keep asking questions, keep responding.