We can't really help you with capacity planning but you may be able to get some pointers about capacity management.
Mysql server stopped two times
It is very unlikely that MySQL just stopped - and even then that's not a meaningful description of why it ceased to function. More likely the daemon was killed by OOM or crashed. There are several things that might have happenned - exactly what did happen is recorded in your log files.
First thing to do is man-up and stop using cpanel for your configuration - such tools can be useful for handling mundane admin tasks but they hide a lot of what's really happening in your system. We need to know how your server is configured. Next you might want to spend some time learning how to manage your applications - go get some good books and spend time on the internet - there's lots of documentation out there.
I found plenty of httpd processes
Not a meaningful metric.
My sites get traffic of 2-3000 unique visitors a day.
Also not a meaningful metric
There are a large number of httpd's in the run queue - there's lots of reasons why that may be the case. Limiting the number of connections/processes/clients will avoid the problem by blocking connections from your users - do you want to provide a good service or keep your system stats sweet?
There is a lot which can be done to improve the performance and throughput of a system (if each requests takes less time then there will be fewer requests in progress at any one time, hence lower CPU and memory demands) - but that needs time, investigation and the skills of someone who knows what they are doing - not a question and some answers on serverfault.com. Alternatively you could just buy more hardware - the screenshot suggests you need around ten times the current capacity (although it may be significantly less depending on the nature of the current performance crisis) - hence hiring a competent HTTP performance expert for a couple of weeks may prove cheaper.
