I know that this question can't be answered in a simple way but I need some advice here.
Has anyone here published an App using MySQL with >10000 users? How many servers do you use? How do you manage "load balancing" and how much does this cost?
I just need a direction how much database/server power one needs for x users. This is important for my future projects. Just imagine I would publish a APP and 100K users or even more would install and use it.
I know that this is a complicated question without knowing details about transaction rates and data traffic but I don't want to get in trouble
Note that 100k users is not the same as 100k concurrent connections.
I don't think that your question can be answered.
One approach is to use a PAAS solution that allows you to scale up the server. Another approach is to start with a simple server and later when you are close to its limits you can switch to a more powerful server.
For that sort of size I would be looking at using a hosted cloud solution. You can then scale it up as required very easily. For ideas on pricing look at someone like Rackspace who are near the top of the market in terms of pricing.