This depends primarily on the size of the space you need to cool. The outdoor component will vary in size, with the largest ones able to connect to up to 8 indoor units. The cooling power of ductless A/Cs can vary from 5,000 BTUs to 50,000+ BTUs. Because every situation and home or building is different, online size estimate charts are unlikely to be of much use. In order to determine the size you need, it’s best to have a free estimate done by an experienced local HVAC contractor, who will do a load calculation, meaning an assessment of your cooling needs based on the size and configuration of your space.