You can create as many sigils you want to at one time, and as often as you like. However, you must always write your intention as though it has already happened, 'I passed my math test with top marks'. Don't use 'I will', always 'I have' or 'I got'. State it as if it exists in the present.
There's another trick you can use: create a sigil for something that is already a thing. Something like, ' I am in my room', or 'I live here'. Group your sigils together when following the directions. The certainty of the extra sigil imparts certainty to the others.
As far as the intention goes, if it's important to you, then it's important enough.