The first commercial fleets of Robo-Taxis will be on the road soon. Today important efforts are made to anticipate future Robo-Taxi services. Fleet size is one of the key parameters considered in the planning phase of service design and configuration. Based on multi-agent approaches, the fleet size can be explored using dynamic demand response simulations. Time and cost are the most common variables considered in such simulation approaches. However, personal taste can affect the demand and consequently the required fleet size. In this paper, we explore the impact of user trust and willingness to use on Robo-Taxi fleet size. This investigation is carried out by simulating the transportation system of the Rouen Normandie metropolitan area in France using MATSim, a multi-agent activity-based simulator. A local survey is made in order to explore the variation of user trust and their willingness to use future Robo-Taxis according to the sociodemographic attributes. After applying this survey data in the simulation, the obtained results reveal the significant importance of traveler trust and willingness to use variations on Robo-Taxi use and the required fleet size.