How can I have 150 or more credits? I mean I want to have '150/150 credits'.
The point of the credits system is to make access to the devices fair so that everyone can take a turn at using them. You cannot get that number of credits as it would mean you could add a lot of jobs to the queue, slowing down response times for everyone else. I don't think there is any simple way to increase the number of credits you have.
I would suggest running jobs on the simulator if you need to run that many credits worth of jobs, as simulator jobs don't use up any credits.