Theory:

The living organisms are millions in number. They are of different shapes and sizes and contain organs which also vary in shape, size and number of cells.
The scientists observe the cells and their parts under the microscope in detail by staining them with dyes.
Cells - Based on number:
The number of cells in living organisms varies from organism to organism. In trees and animals like elephants, there may be billions and trillions of cells. The Human body has trillions of cells which vary in shapes and sizes. These groups of cells perform various functions.
Important!
A billion is a thousand million, and one trillion is a thousand billion.
The living organisms are classified into two different categories based on the number of cells.
  
Unicellular organisms:
  
The living organisms which are made up of only one cell are known as unicellular organisms. These organisms are generally microscopic and hence can be seen only under a microscope. The single-cell found in these organisms performs all the necessary functions that are required for the survival of the organisms.
Example:
Ameoba and paramecium are examples of single-celled organism.
 
512px-Amoeba_(PSF).svg.png
 Amoeba
 
 1024px-Paramecium_diagram.svg.png
Paramecium
 
These organisms perform various functions such as digestion, respiration, excretion and reproduction similar to the multicellular organisms that contain groups of specialised cells forming different tissues which form organs.
 
Multicellular organisms:
  
The organisms that are made up of more than one cell are known as multicellular organisms. These type of organisms are in general macroscopic in size and can hence be seen with the naked eye.
 
1024px-Figure_19_00_01.jpg
Multicellular organisms
Even though the number of cells may vary between two different organisms, it will not affect the functioning of the living organisms. The living organisms that comprise of a billion cells begins its life as a single cell which is the fertilized egg. This egg multiplies and increases the number of cells.
Reference:
https://upload.wikimedia.org/wikipedia/commons/thumb/9/9c/Amoeba_%28PSF%29.svg/512px-Amoeba_%28PSF%29.svg.png
https://upload.wikimedia.org/wikipedia/commons/thumb/f/f8/Paramecium_diagram.svg/1024px-Paramecium_diagram.svg.png
https://upload.wikimedia.org/wikipedia/commons/thumb/b/b1/Figure_19_00_01.jpg/1024px-Figure_19_00_01.jpg