You can eat as many M&Ms as you want to a day.
Although you should limit the amount of M&Ms you eat a day to a few handfuls as they cane be unhealthy, especially if you're not eating a healthy and balanced diet.
Eating too much candy including M&Ms in a day can cause stomach upset, stomach cramping and bloating.
You should aim to limit added sugars, like the added sugars in M&Ms and other candy to less than 10% of your total daily calories.
If you're on a 2,000 calorie diet for example, this means that you should eat no more than 200 calories from the added sugar which means that you could eat around 17 plain M&Ms a day.
Eating up to 17 to 20 M&Ms a day is generally okay.
Just be sure to eat more healthy foods and not just eat candy.
Candy is okay to eat in moderation but it should not be eaten as your main food source.
The body needs other healthy foods to remain healthy and give you energy.