It's challenging to provide a specific number of people in North Africa who modified the environment for farming as this information is not consistently documented and can vary widely over time and by region. However, throughout history, many societies in North Africa, including in areas like the Nile Valley, the Sahara, and the Maghreb, have engaged in various agricultural practices that required significant environmental modifications. These modifications have included the development of irrigation systems, terracing, and the cultivation of specific crop types suited to local conditions.
If you are looking for detailed statistics or numbers for a particular area or time period, more context would be helpful, as well as access to specific agricultural studies or census data which might shed light on the scale of agricultural modification in certain regions or communities within North Africa.