There are different ways to achieve this, but for me the easiest and quickest is to use the "," custom specifier
double value = 1234567890;
// Displays 1,234,567,890
Console.WriteLine(value.ToString("#,#", CultureInfo.InvariantCulture));
// Displays 1,234,568K
Console.WriteLine(value.ToString("#,##0,K", CultureInfo.InvariantCulture));
// Displays 1,235M
Console.WriteLine(value.ToString("#,##0,,M", CultureInfo.InvariantCulture));
// Displays 1B
Console.WriteLine(value.ToString("#,##0,,,B", CultureInfo.InvariantCulture));
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