Parallel world is a messy concept... If a person upon making a decision, will have created another parallel world with the decision not been taken... If there are 4 decisions that that person had to make (each with only 2 possibility), then there will be 2^4=16 parallel world. If there are 2 person in the world and each person have 4 decision to make, there will be 16x16=256. If we assume that the 4 decisions are made in a day, in each person lifetime and how it interact with other's decision, there will be trillion or more worlds (possibly infinite)