1) I have invited twenty five people. I don't know if twenty will show up. I am sure the other five will be here.
2) I have invited twenty five people. I don't know if twenty will show up. We may have fewer than twenty guests.
3) I have invited twenty five people. I don't know that twenty will show up. We may have fewer than twenty guests.
Do these sentences work?
In '1', I have twenty specific people in mind. I don't know if those twenty people will show up or not.
In '2' and '3' I am not sure that even twenty people will show up.
I guess in this situation it is possible that all of them will show up.
If you hyphenate twenty-five, all three are fine.
You can also use digits: 25. There's not much agreement on when to spell out numbers and when to use digits, but in this case, either would be fine with most editors.
I'm not a teacher. I speak American English. I've tutored writing at the University of Southern Maine and have done a good deal of copy editing and writing, occasionally for publication.