3CX is priced as an annual licence/subscription which is based on the number of simultaneous calls required. As a rule of thumb, 1 sim. call per 3-4 extensions is sufficient, i.e. a company with 50 extensions would need a 16 sim. call licence. You can upgrade your subscription at any point in time by just paying the difference for the remaining period. If you prefer to buy a perpetual license and pay yearly maintenance.