Mozart had many famous symphonies, but the last three - often referred to as the 'final trilogy' - are regarded by many as his greatest symphonies. These were written in 1788, in the summer before...
there are 41 of them but the most famous ones are the Haffner, Linz, and the Jupiter
His 40th symphony is believed to be the heartbreak and sorrow he felt after losing his fifth or sixth child (Can't remember). That's all I know.
Symphony No.39 K543 does not have a specific name
He was able to compose his first symphony at about the age of 5, from there on he composed till December 5th his death day.