Scaling is real beast of problem, deal with it as you grow.
Most aproachs deal with it separating different layers in your solution, your application is just one layer.
Probably the first layers (user perspective) is load balancing, there are several ways to do it, to use Xojo Web framework you have to use session stickiness. Why ? Because with multiple copies of the same app, session cookies are not shared. You could roll your own shared cookie solution, basiclly save the cookies to database and accessit by all copies, but really is a project on it’s own.
Next is securing the conection, TLS (from Https), this is cpu heavy and should be handle separatly. Probably good ideia to handle it on the same machine (vm) thats doing load-balancing, Nginx or Haproxy can handle it.
Next layer is the application itself, do some profilling to detect which functions/features are most used and improve performance (code refractoring). Logs are your friend, log everything to determine comun usage profile and optimize targeting the usage profile. If you application is healy used some days of the week or month, think about scalling (adding vm or copies of your app) just for that period of time.
Propabilly the next layer is the database, most commun in web apps, scalling databases is not your run of mill activity. You can grow database cluster but is really advance/specific topic. Easier is just get bigger machine to handle the database. Most web apps stress network conections and memory, databases stress I/O disk access. Again knowing your app usage can help, sizing data and queries helps to optimize workloads and response time.
Web apps are all about responsiviness to user interactions, nothing is more frustrating than clicking a button and wait for minutes.
Improving is a continuos job, one feature at a time.
One last word, lost of engenieers have work on this problem. Virtual machines and containers are basic building blocks of todays web architrecture, don’t try to learn everything to do it your self. Just luch the product/app based in cloud offer (maybe xojo cloud) and learn users usage patterns and data consuption, then use this knowledge to scale the app to more users. Early optimization is evil in pure form, costs to much time and money.