There are a couple of ways to do it:
1) Read the file as a string, and use string.Split to break it into numbers:
string s = File.ReadAllText(path);
string numbers = s.Split(',');
You can then loop through each number and use int.Parse to convert the string value into an integer, and store it in an array or a List<int>
2) Treat the file as Comma Separated Values, and use one of the many converters available. Here is one: A Fast CSV Reader
The first is simple to implement, the second takes a little more work. But it's worth considering the second, because it may be easier to use in the future, should your data change for any reason.