2,000,000,000
2,500,000,000
To write 2.1 billion in digits, you would write it as 2,100,000,000. This is because the number 2.1 billion consists of the numeral 2 followed by nine zeros, representing the value of one billion.
The number 2 billion can be written as the Roman numeral [MM] with a horizontal bar above it (which I can't write with my keyboard).
Since you asked how I would write the number, the answer is 1031.
As a number: 7,125,000,000 = 7.125 billion
As a number it is: 24,600,000,000 = 24.6 billion
As a number it is 12,800,000,000 = 12.8 billion
19.35 billion = 19,350,000,000
As a number it is: 529,600,000,000 = 529.6 billion
As a number it is: 6,080,000,000 = 6.08 billion
As a number it is: 1,800,000,000 = 1.8 billion
As number it is: 1,500,000,000