There have been various attempts to gather data on medical travel and medical tourism numbers. The problem is that it's not very easy and many have got it badly wrong through not understanding the market, or by counting the wrong thing. There are also various inflated medical tourism numbers and projections floating around which are "invented" by those who have an interest in promoting the industry.
Major studies have been attempted by both KcKinsey and Deloitte but they measure medical travel in different ways. There's also a Medical Tourism Facts And Figures Report that may be of use to you.
You mention counting the 'number of arrivals' to a hospital i.e. admissions. What you count creates problems of comparison across countries. For example, a hospital in Asia may claim very big numbers for international patients but when you investigate further you find that:
- patients get counted many times over. e.g. a patient has a consultation, a scan, the operation and some post operative treatment... and gets counted four times.
- a patient may be counted as an international patient or medical tourist when in fact they are a long term resident of the country.