Well here is an example from the UK, analysis from 2012, found that there were

• 2,000 a year do the GCSE Astronomy course (exam typically taken at age 16 but sometimes by adults)
• Over 10,000 amateur astronomers judging by membership of the 200 astronomy societies.
• 3,000 members of the British Astronomical Association (BAA)
• Similar number of memberse of the Society for Popular Astronomy (SPA)
• Sky at Night and Astronomy Now magazines circulation in the mid 20,000s
• 618 paid positions in universities for astronomy including technical staff - that includes 227 professors and 160 senior lecturers.
• 700 acceptances of undergraduates per year to a class with some astronomy component (e.g. physics including astronomy)

From: http://www.venngeist.org/opsa1_m...

Then in the US:

• Astronomical League has membership of 13,000
• Estimated 200,000 to 500,000 amateur astronomers

https://astrosociety.org/edu/res...

The total number of individual members of the International Astronomical Union is 10,190 - gives a rough idea of the number of serious professional astronomers though many astronomers are not in the IAU. As you’ll see from the list, they come from many different countries. International Astronomical Union | IAU

Must be hundreds of thousands to millions of amateur astronomers world wide from the US and UK figures.