Hi @stefanidi,
You can find in this post some related links: Number of simultaneous users on open edx
Also, in this wiki page you can find more info about edx.org as of 2015: https://openedx.atlassian.net/wiki/spaces/OpenOPS/pages/60228120/Running+Open+EdX+in+Production#RunningOpenEdXinProduction-edX.orgDeployment
Probably what may be useful to estimate is how “concurrent” those 150K enrolled users are. How many courses do you have, if the course are self paced or not, etc.
As @regis mentioned, it would be extremely valuable to have your feedback.