Yes during a CT scan you have to hold your breath until the CT scan is over.
The reason you need to hold your breath is because when you breathe in and out and your body moves it can mess up the results of the CT scan.
So during the CT scan you'll be asked to be very still and hold your breath until the CT scan is over.
The CT scan itself takes only a few seconds usually but sometimes it may take up to a minute or two.
Most people should be able to hold their breath for a minute or two but the last time I had a CT scan done a few years ago I was asked to hold my breath and stay as still as possible and it only took around 10 seconds from start to finish for the CT scan.