Recommended approach for form errors

I am in a bit of confused state in terms of how to design flows for forms.

Let's consider a scenario

I have a form with fifteen fields(it is pretty normal in my application). All these fields have their respective validations and corresponding error messages.

Now given that there is a high probability that there can be multiple erroneous fields, there will be multiple error messages returned from the API. I also have control over how the API returns those errors. Where I am stuck right now is do I return only first error encountered or do I process the entire form and then return the set of errors. For me both of them have it's pros and cons.

But I am unsure of how others deal with it in general. What would be a good practice? Would it be to process the entire form and then return set of errors in one go to save consequent multiple requests to complete the cycle of correction-submission-validation or just return the first encountered error?