Re: Life goes on
They could well have the same client for a long period of time (months) if working on a large job. They may well invoice monthly, or weekly, or on completion of key deliverables. None of these are indications of whether they are running a business.
I have known consultancies with multiple employees who have only one client which they invoice on a monthly basis. Are they in business, or are all their employees actually employees of the client? What about cleaners contracted through an agency, should they be classed as employees of the client?
I will add again: The courts and HMRC themselves have said that number of clients has no bearing on IR35. Indeed, you can have some contracts inside and some outside at the same time.