it is difficult to provide an exact number of how many quantum computers existed specifically in 2018. Quantum computers are still in the early stages of development and are not as prevalent as classical computers.
During that time, quantum computers were primarily being developed by several major companies, research institutions, and startups. Companies like IBM, Google, Microsoft, and Rigetti were actively involved in building and advancing quantum computing technologies. Additionally, research labs and academic institutions around the world were also making progress in developing quantum computers.
It's important to note that quantum computers are highly specialized and require unique conditions to operate, such as extremely low temperatures and isolation from environmental disturbances. Consequently, their numbers have been limited, and they are typically not as widespread as classical computers.
Given the dynamic nature of the field and the pace of advancements, it is advisable to consult more recent sources or news updates to obtain the most accurate and up-to-date information on the current state of quantum computers.