We herein investigated the effect of the number of alkoxy chains on the two-dimensional self-assembly of a trigonal molecular building block. To this end, a dehydrobenzoannulene (DBA) derivative, DBA-OC14-OC1 having three tetradecyloxy chains and three methoxy groups in an alternating manner, was synthesized. Scanning tunneling microscopy (STM) observations at the 1,2,4-trichlorobenzene (TCB)/graphite interface revealed that DBA-OC14-OC1 forms three monolayer structures: Porous honeycomb, parallelogram, and hexagonal A structures. At the 1-phenyloctane (PO)/graphite interface, DBA-OC14-OC1 also forms three structures: Parallelogram, hexagonal B, and dense linear structures. In contrast to the previously reported DBA derivative DBA-OC14 having six tetradecyloxy chains, DBA-OC14-OC1 shows structural polymorphism with a variety of alkyl chain interaction modes. Because in the observed patterns DBA-OC14-OC1 adopts a low symmetric Cs or C1 geometry, the variation of the interaction modes and the resulting network patterns likely originate from the conformational flexibility on surface of this DBA by decreasing the number of alkyl chains. Molecular mechanics simulations gave insight into the structural and energy aspects of this pattern formation. We also discussed the role of solvents, TCB and PO, on the polymorph formation.