My google-fu turned up this:

http://www.gardening-resources.com/growing-flowering-cherry-trees/

Which says you need to keep seedlings at least 10ft apart.  That is a mere 3 meters.  If you have enough space for 12 meters for 12 trees, that ought to be MORE than enough.

If you are wanting to know about parties under them, think of it this way:

1 tree with 12 meters radius perimeter.  Thinking of it as a grid, you could put down about 2 5-meter wide blue tarps on two of the opposing compass points.  The next tree over could do the same.  That would give you 2 meter pathway (gauntlet) to walk through with room for people to line up their shoes and still give out decent cat calls at the other poor sods that didn't manage to stake their places out in early March.