Linux: Increase capacity of current server Vs add new more server

Question

I have one website with below configurations:

• 2 Web Servers with Load Balance enabled (Each Server having - OS: Linux, Storage: 30GB [20% Used] and RAM: 10GB [44% Used])
• 1 Database Servers (Storage: 30GB [20% Used] and RAM: 10GB [44% Used])

And with current configuration its able to handle 5000 concurrent users without any problem but in future more users (1000 to 1500) are supposed to access the website.

So to handle more user volume I figured out two options:

1. Increase capacity of current infrastructure by 25%
2. Add new web server same as current capacity

But I'm little bit confused about what is the difference between point# 1 and point# 2 irrespective of the cost.

Answer 1

Points #1 & #2 represent Vertical & Horizontal scaling of resources respectively.

Vertical scaling (Increasing of resources) is useful whenever your application needs more computing resources to process the request - Mostly used while running resource-intensive workloads.

Horizontal scaling (Adding identical servers) is useful for handling more requests because requests are distributed across available servers also this will be useful for the High Availability (HA) setup.