The wearing of cap numbers on the shirt under the team crest was adopted by Australia in about 2001, and is gradually being adopted by other nations. I have seen Australia, India, England, and Sri Lanka all do it, but not sure about others, no doubt they will follow as it is a huge honour to play test cricket for your country.
Interestingly, although Australia and England both played in the first ever test match, there is a huge disparity in the number of representatives, Australia's most recent test cap, Dan Cullen received Baggy Green, number 397 at Chittagong (CDS) on the 16th of April, 2006. England's most recent debutant, Jon Lewis, received his Three Lions number 634 on the 2nd of June, 2006 in Nottingham vs Sri Lanka. England have used nearly 240 more players in the nearly 140 years of test match cricket. Australia's tally has two controversies: The first being that the first official tour of the UK by an Australian team, was made entirely by Indigenous cricketers, who were not granted full test recognition, and the second, Brendon Julian and Michael Slater swapped numbers after Slater had the wrong number tattooed upon his chest.
Numbers by country:
England: 634
Australia: 397
South Africa: 299
West Indies: 265
India: 256
New Zealand: 233
Pakistan: 186
Sri Lanka: 104
Zimbabwe: 74
Bangladesh: 45