Size and complexity of the operator - Reference: Reg. (EU) No 965/2012 on Air Operations: ORO.GEN.200 (b)
AMC1 ORO.GEN.200 (b) paragraph (a) defines how to assess if an operator is complex or non-complex:
The operator is non-complex if its workforce is less than 20 full time equivalents (FTEs). However, point AMC1 ORO.GEN.200 (b) paragraph (b) indicates that an operator with less than 20 FTEs may also be considered complex if, for example, it performs high-risk commercial SPO or operates in a challenging environment (offshore, mountainous area, etc.).
Prior to sending a declaration an operator should check with the competent authority, if their assessment of complexity is correct.