According to AMC1 ORA.GEN.200(b), the complexity of an organisation depends primarily on the number of full time equivalent employees. Organisations may also be considered to be complex based on the extent and scope of contracted activities subject to the approval, certain named risk criteria, different types of aircraft used and the environment (offshore, mountainous, etc.).
There is no provision for an ATO to be considered non-complex on the basis of its non-contracted activities and, therefore, any ATO with fewer than 20 FTE employees should be considered non-complex. However, this may not apply in the UK since the CAA now seems to make up the rules as it goes along.