English speaking countries use the short scale billion, with 9 zeros: 1,000,000,000
Non-English speaking countries largely use the long scale billion, with 12 zeros: 1,000,000,000,000.
Britain used to use the long scale, but changed in 1974 to the short scale billion.
It has 21 zeros, because 'Giga' means 'a Billion'. Thus, one GigaTrillion is 1 Billion-Trillion. A billion has 9 zeros, and a trillion has 12 zeros. However, if you are referring to the United Kingdom, a billion has 12 zeros and a trillion has 18 zeros, so it would be 30 zeros.
9
In English speaking countries there are 9 zeros in four billion. 4,000,000,000 In other countries there may be 12 zeros in four billion. 4,000,000,000,000
In English speaking countries there are 9 zeros in five billion. 5,000,000,000 In other countries there may be 12 zeros in five billion. 5,000,000,000,000
In English speaking countries there are 9 zeros in seven billion. 7,000,000,000 In other countries there may be 12 zeros in seven billion. 7,000,000,000,000