A business return needs to include everything for that business.

Getting a 1099-NEC or not, has nothing to do with it.

Getting 1099-K has nothing to do with it, either.

1099-NEC is issued by your client's customers when that customer, as part of being in business, had your taxpayer do something "business-like" for them and it reaches $600 or more in the calendar year, so they report to the IRS that your client is "in business" and the IRS now knows to expect a tax return from your client.

1099-K is issued because of the movement of money. That doesn't make it business, or any part of it business, but it also is possible all of it was business.

So, your client could have grossed $3 million in business that year, but if no one client paid $600 or more, they would never see a 1099-NEC. And, if they only paid in cash or checks, there would never be any 1099-K, either.

That's why the business records everything for the business that happens in the business, and that is the data you use for the tax return. If you are handed multiple 1099-NEC, add them up. Your client had better be reporting at least that much in Gross.