I am using lazarus and Firebird 3.03 to develop a project for an ISP which has approximately 15,000 users which are managed by 136 Mikrotik Routerboards.
The project consists of 8 programs which fulfill functions from the request for internet service, scheduling of activities, creation of user subscriptions, billing, activation of internet service, service outages, tickets, inventory, among others.
The programs are used 24/7 and can have up to 100 users working at the same time.