Thanks for your replys!
Yes i do read the posts, but i have been really bussy with my school project that i had to present yesterday.
I had 3 benches with 3 meters in between and i ended up with the easy (for me) solution with a wire for each button (18 in total) and a common ground for each set of 6 buttons. It worked out pretty well.
Im thinking about doing a more permanent version for outdoor use (i have applied for some founding for materials) and if im doing that, i will look into a smarter solution.
The buttons on the benches registres when people sit and controls the leds, that was devided into sets of 9. This is my first project involving electricity ever, so i think it turned out pretty cool - but it would never bee possible for me to do without the help from this forum and tuts from instructables - so thank you!
Some photos from the project: