You should always state a 24 hour cancellation policy!
Since it is your own business you have the right to “overrule” that policy and not charge if an emergency came up. These policies are implemented to make a standard across the board, keep your business going strong (you don’t have to try and make up the hours elsewhere), and to keep the trainer in “control”.
If you build solid relationships with your clients you are less likely to have cancellation problems. However, especially just starting out you want to make sure that you are not sacrificing your time and finances for repeat offenders. This can quickly lead to you manipulating your personal schedule for the clients needs which ultimately lead to burnout.