If you experience a flat, the first step is to safely pull over to the side of the road.  At this point you have two options.  Option one is to wait for support from the race organizers who normally have mechanics on course travelling on motorcycles.  If you are extremely fortunate and there is a mechanic close by, you will be back racing in minutes.  This, however, is very unlikely and you might end up waiting for a very long time.  Option two is to fix the flat yourself.  You should practice this at home before you leave for any race so that you can improve and limit the time required to perform the task.  There are plenty of videos all over the internet to guide you through the procedure.  If done well, you should expect to lose no more than 5-10 minutes.  Make sure that you have the supplies you will need attached to your bike in a saddle bag, bento box, or taped under your seat.  You will need at least one spare tube (getting more than one flat is extremely rare), a set of tire pullers, carbon dioxide cartridges (bring two) and an adapter to attach to the cartridges to the valve of your tube.