If you are sure you don't want to use arrays then you can try something like simply placing the numbers together in a string. You can then iterate through each character in the string and keep track of the count through a loop.
psudo code:
private string allData;
public void GetData(string theNumber)
{
allData += theNumber.ToString;
}
public string MostCounted()
{
string currChar;
int currCount;
string highestCountedChar;
int highestCharCount;
for (int i = 0; i <= allData.Length - 1; i++) {
currChar = allData.Substring(i, 1);
for (int i = 0; i <= allData.Length - 1; i++) {
if (currChar == allData.Substring(i, 1)) {
currCount += 1;
}
}
if (currCount > highestCharCount) {
highestCountedChar = currChar;
highestCharCount = currCount;
}
currCount = 0;
}
return highestCountedChar;
}