How many times of day one can say viduy/ mekae teshuva?

For most of those who say viduy every day, the custom is once in the morning and once in the afternoon, and sefaradim say in Kriat Shma before going to sleep, as well. but if you feel a need to say more (e.g. after you made a sin, even accidentally) you can do so as often as you need- just try to be as sincere as possible.