Event Timelines can be created to be Time specific or Sequence specific.
An Event Timeline that's based on time allows your customer to specify the exact time a certain activity will take place. (Several Event Timelines based on time-specific events are included automatically in all accounts.)
An Event Timeline that's based on sequence is less specific on when something will happen, but rather the order in which activities will happen. The client can drag each item around for ordering.
There is no limit to the number of timelines you can create in either format.