Sir Don Bradman, an Australian cricketer, was the first person to convert his maiden Test century into a double century. He achieved this feat on February 3, 1930, during a Test match against England in Adelaide. Bradman scored 112 in the first innings and 299 not out in the second innings.

Since then, many cricketers have converted their maiden Test centuries into double centuries. It's challenging to provide an exact number since this statistic can change with every new Test century scored. However, I can provide some notable examples of players who have achieved this milestone after Bradman. 

Some well-known players who have converted their maiden Test centuries into double centuries include:

1-  Sir Don Bradman (Australia):

Sir Don Bradman, often regarded as the greatest batsman in the history of cricket, had a remarkable career.

  • Maiden Century: 112
  • Double Century: 299* (vs. England, Adelaide, 1930)
  • His maiden Test century was a score of 112, which was a significant achievement in itself.
  • However, his most famous innings was his double century, an incredible 299* not out, which he achieved against England in Adelaide in 1930.
  • Bradman's batting average of 99.94 in Test cricket remains unparalleled and is a testament to his extraordinary skill and consistency.

2-  Vinod Kambli (India): 

Vinod Kambli was a left-handed batsman who played for the Indian cricket team in the 1990s.

  • Maiden Century: 224
  • Double Century: 224 (vs. England, Mumbai, 1993)
  • He scored a double century (224) against England in 1993, which was his second Test century.
  • He scored his maiden Test century with a brilliant 224 against England in Mumbai in 1993.
  • Notably, Kambli's double century in the same match was also 224, making it a remarkable achievement in his career.
  • His double century was a crucial contribution to India's victory in that Test.

3-  Brendon McCullum (New Zealand): 

Brendon McCullum, born in 1981, was a dynamic cricketer who excelled as a batsman and later became one of the most successful captains for New Zealand.

  • McCullum scored a double century (225) against India in 2010, which was his second Test century.
  • Maiden Century: 225
  • Double Century: 225 (vs. India, Hyderabad, 2010)
  • McCullum's maiden century of 225 came against India in Hyderabad in 2010. It showcased his aggressive style of play and fearless approach to batting.
  • He scored a double century (also 225) in the same match, making it his second Test century. McCullum's leadership and contributions significantly elevated New Zealand cricket.

4-  Azhar Ali (Pakistan): 

Azhar Ali is a Pakistani cricketer known for his solid batting technique and resilience.

  • Azhar Ali scored a double century (200*) against West Indies in 2016, which was his second Test century.
  • Maiden Century: 200*
  • Double Century: 200* (vs. West Indies, Abu Dhabi, 2016)
  • His maiden Test century was a significant 200* against the West Indies in Abu Dhabi in 2016.
  • This double century was not only a personal milestone but also a crucial contribution to Pakistan's success in the Test match.
  • Azhar Ali's ability to occupy the crease and accumulate runs made him a vital part of Pakistan's Test team.

5-  Bruce Mitchell (South Africa):

Bruce Mitchell, who played for South Africa, was born in 1914 and was known for his elegant batting.

  • Maiden Century: 189
  • Double Century: 189 (vs. England, Leeds, 1929)
  • His maiden century of 189 came against England in Leeds in 1929. While he didn't score a double century in his Test career, Mitchell's contributions were valuable to South African cricket.

6-  John Edrich (England):

John Edrich, born in 1937, was an English cricketer known for his gritty batting style.

  • Maiden Century: 120
  • Double Century: 310* (vs. New Zealand, Leeds, 1965)
  • His maiden century of 120 was followed by a remarkable double century of 310* against New Zealand in Leeds in 1965. This innings is a testament to his determination and resilience as a batsman.
  • Edrich enjoyed a successful international career and played a key role in England's batting lineup during his era.

In conclusion, these cricketers left a lasting impact on the game of cricket through their remarkable performances. From Sir Don Bradman's unparalleled batting average to Vinod Kambli's stylish strokeplay and Brendon McCullum's fearless approach, each player brought their unique style and skill to the sport. Azhar Ali's patience, Bruce Mitchell's elegance, and John Edrich's determination also contributed significantly to their respective teams. These cricketers continue to be remembered for their contributions to the rich history of Test cricket.
These are just a few examples, and there have been more instances of players achieving this milestone. To get an exact count of how many times this has happened, you would need to review the complete historical records of Test cricket, which is continually updated with new matches and centuries scored by different players.