Is this correct? i am thinking maybe it is bastantes?

Demasiados

so you want to know what means what.

Your stuck between Demasiados y Bastantes

These two are very go chooses but they have slightly different meanings

Demasiados means too many

bastantes means enough

looking at the sentence

it says "If you ONLY(key word) need one pencil and someone gives you 20, you have...enough or too many pencils?

If you only needed one Think about it you ONLY NEED ONE but now you have 20 You dont need the other 19 pencils so ANSWER WILL BE

TOO MANY

you have too many pencils becuase u only need one