There's more than 140 of them. Each area has its own set number (you can see the link at the bottom of the Wiki for Town Hall Quests). It's a little out of date as I think a few more were added the last update or so.
In general it seems, Town Hall Quests are randomly rolled to see if a Town Hall will have a quest, than randomly rolled which one.
There's nothing for completing all the Town Hall Quests. You earn an achievement for completing 5, 15, 40 Town Hall Quests (Public Service I, II, III).
Town Hall Quests can help get the reputation needed for some quests or in-game actions such as buying an house. At one time, Town Hall Quests were useful to help gain levels and gold when a person was struggling. Since the last few updates made the game a little easier (less XP for level up, damage boost etc), Town Hall Quests are more of an optional player thing than a required part of the game.