I'm thinking of not giving up my seat
Why won't you give it up?
There's no other seats available and I want to sit down.
I understand you wanting to sit down but this isn't even your event. They need all the chairs for their party so you should give the seat up. If someone else needs a seat is usually the right thing to let them have it.
It's up to me when to get up, I don't want to move.
If you weren't invited you should respect the hosts. Most of the time people order the exact number of seats they might need.