These limits strictly depends on hardware choice and LAMP included services configurations and application architecture. Need more information before make right suggestions.

Hi Egils, I mean with the same hardware and configuration. 

I red that Apache Web Server is slow when execute some queries, and Lighttpd or Nginx are faster. 

PHP is the better choice? what is AJAX ? and MySQL GPL is able to manage a lot of users with simultaneous access (100k or more?) or is better to use MySQL Enterprise Edition when the number of users grow?

I developed a web system based on LAMP, with about 5k users, but the same architecture could run with 100k users or more?

Thank you for your reply

if your websystem is built keeping in mind all kind of scaling, then yes.

I suggest make some measurements of internal and external websystem traffic load, node CPU load , mysql DB load. Maybe that helps you get light in. You need to find the bottleneck of your websystem. If You got it You can increase the performance of websystem. Try to bring Your test on one of the websystem node. Then You`ll see the behavior of the websystem node, how many requests the node can serve, etc.

The should look into horizontal scaling of your system.

One solution could be to employ HAProxy load balancer to many same PHP application servers. 

When scaling your database, it is a bit more tricky. For scaling the reading from MySQL you can employ a single Master (for write) with multiple Slaves (for reading). But this will scale only reading which is OK for most websites.