How many strollers should I get for my baby?

I got at least 3 different types as i got 4 kids. Both single stroller and double strollers. Basically, all will cater for different occasions. Single stroller (lightweight) for travelling on buses. Single heavy duty for long walk Double stroller side by side type to bring 2 kids out - good when both want to sleep. Front and back diuble stroller good when travelling with 2 kids and yet don't wanna take up too much space.