How do you scientists count a species population?

Well, a species could be found over a wide area... perhaps even worldwide, so you would have to either divide into geographical areas and count the members of the species only in that area, or you would have to do something more inventive. For humans, we do a lot of census polls. For animals we often catch wild animals and plant trackers in order to get a better idea of where members of the species are and get a good count. Other animals, like pets, we can get good estimates from other sources like breeding records, reports of feral animals, pet store sales, etc. No method can be perfect unless the entire population is very small and in one place.