Does someone know of a website that keeps track of a species population?
How about human?

You will find the data of human population growth on the webpage "Our World in Data". Scroll to the second graph, click on the tab "data" and download the .csv file.

Just google for any specific species of interest

Otherwise you can just google things of interest. Look for large emblematic mammals or commercially important species.


If you google "bald eagle population growth", you will easily find these data.
If you google "moose growth population breeding pairs", you will find these data.
If you google "killer whale population growth", you will find these data.
If you google "tuna population growth", you will find these data.


COMADRE

COMADRE is a database of animal matrix population models. It can be of interest to you.