In Argentina:
- we use 1 mn (you can use 2 mns, but that's not common... as it's not common to use only a fn)
- we only use a few composed names (María X, Juan Manuel, Juan Ignacio, Juan José... I guess it's the same in Spain, at least for men, from what I've heard)
- we use our father's surname unless it's too common, short or something else