The minimum for each primary statistic is 1 and the maximum is 10. These boundaries can never be broken, even with bonuses or maluses, although the game keeps track of further increases internally. For example, if someone has 10 Agility and then finds the bobblehead for it, the Agility still stays at 10, but if there ever is a negative modifier - e.g. wearing metal armor (Ag -1) - the Agility will stay at 10, because it was technically 11.