They’re very easy to count, you can take a drop of blood, spread it on a glass slide and look at it under a microscope. There are more advances methods too, using flow cytometers for example.
It’s a very standard test in medical laboratories.
A human adult has about 25 trillion red blood cells (about 5 trillion per litre of blood), and 30-50 billion white blood cells. The numbers are not fixed, they vary from one person to another and over time, especially during infections.
When you get your blood tested, the results will give a reference range, if you are healthy then your numbers should fall within that range.