I can find a lot of information on the number of individuals with dementia, globally, but cannot seem to locate how many informal caregivers there are worldwide.
Though it might be hard to find a number for informal caregivers for dementia globally, to get a general idea you could look into countries that have done extensive research on the topic. There are a lot of studies from Canada on informal caregivers for people with dementia. Data from the US and Canada illustrate the significance of the issue pretty well. You could gather data from various countries and include that in your research and still emphasize the issue surrounding it.
I'm not sure about the generalizability globally though, since it can vary from country to country as Francesco had already mentioned. But definitely not impossible to gather data on each country and still highlight your point.
I am now looking into compiling some data on caregivers in Spain, Italy, France, and Sweden (age, gender, hours of care, years of care - as much as I can find out). I would appreciate references to specific information if anyone has some.
I can find quite a bit of information on caregivers in Canada and USA, thank you Keshini. I am surprised to learn that Sweden does not really gather information on caregiver age, gender, hours spent in care, etc. (or at least not that I can find in English or my beginner level of Swedish).