depending in the question or focus that you have to respond to will indicate how many texts you should be using in the essay. Keep in mind though that if you focus on 1 set text it doesnt mean you cant pull info in from another, especially if it likes nicely!! make comparisons bewteen set texts and other texts, and if your still not sure NEVER EVER b afraid to hand in a draft n get a teacher to read through it. if you dont like yout teacher for a specific subject find 1 that you do like in the same facalty.
hope this help!