It depends on who made them.
If they are homemade it is safe to assume that the amount of THC will be inconsistent and can be quite large. If you approach all edibles with the mindset of "I can always add more, but I cannot take any away after I eat it," you will have a better time. If they are professionally made and has accurate testing information try to take about 5mgs.
Allow for two hours to take effect and if you decide to take more, take small portions. If you do accidentally take too much, drink lots of water and try to stay calm and comfortable. Common side effects like heart palpitations and drops in blood pressure can provoke anxiety but are not dangerous unless you have preexisting conditions.
I hope this was helpful!