Jmeter-How to record AJAX calls

There isn't a good number for all those parameters, other than as high as possible for your particular testing subject machine. Just load up as many concurrent connections you can with Jmeter (toying with the balance of instances and threads), and fire to your machine. Do so until a certain component (ie MySQL) fails, then try to optimize that component and fire again.

The ramp-up time is not really important except for special cases, so they recommend in the same paragraph to keep it at the default value of 1 sec, which means all threads you enter in the number of threads field will be started within 1 second. Also, for the local OS it doesn't matter at all if you start all threads at once or with a ramp-up time, the limiting factor will be the number of threads alone.

How large you can select the number of threads value on the client machine is dependend on many factors and you have to test it yourself, but since the purpose of JMeter is to see how your server handles X concurrent connections, I assume in many cases the server or the network connection to the server will be the limiting factor, not the local machine itself.