We recommend that you apply to at least five graduate programs. You should always try for the best, so we suggest applying to at least one school that you would truly like to attend, even if you think you might not be qualified. Then two or three high quality programs for which you are qualified. Then one or two that you know you can get into. The number to which you apply can also depends on the time and money you have.
Time is also an issue.
Most applications require an essay, but if you write one essay, you can usually adapt it to others. We don't mean to be insulting, but there are some common sense things about the application forms you fill out. This includes the address part. Whatever you do, don't use a pencil. Give your references plenty of time to send in their letters or forms.