As long as you meet the Home Office’s requirements to be eligible to apply for indefinite leave to remain –including with regard to investment, business activity and job creation –you will be permitted to lodge an ILR application, regardless of your business’s number of customers/clients.
Please note, however, that as a holder of an Entrepreneur visa, you are not permitted to yourself work directly for clients. Instead, your business would need to be engaged by your client, with your services being supplied through your company.