Your regular expression should be \btrue\b to get around the 'miscontrue' issue Casper brings up. The full solution would look like this:
string searchText = "7,true,NA,false:67,false,NA,false:5,false,NA,false:5,false,NA,false";
string regexPattern = @"\btrue\b";
int numberOfTrues = Regex.Matches(searchText, regexPattern).Count;
Make sure the System.Text.RegularExpressions namespace is included at the top of the file.