Here's the top 25. Note that there are approximately 35 more bizjets operating in Aus, but not on the VH register. Some countries (like ships) are using registries of convenience, like Austria is for Russia, And Hong Kong and Macau for China
Position Registered Country No. of Aircraft
1 United States 12,051
2 Brazil 764
3 Mexico 704
4 Canada 483
5 Germany 387
6 Isle of Man 264
7 Austria 244
8 United Kingdom 241
9 China 203
10 Venezuela 168
11 South Africa 160
12 Australia 154
13 Argentina 142
14 India 140
15 Portugal 138
16 France 132
17 Switzerland 123
18 Bermuda 117
19 Cayman Islands 114
20 Turkey 110
21 Spain 100
22 Italy 99
23 Denmark 62
24 United Arab Emirates 61
25 Saudi Arabia 56