Acholi belongs to Western Nilotic network of Luo languages. Acholi is predominantly spoken in northern Uganda in the districts of Amuru, Gulu, Kitgum and Pader. Five languages spoken in nearby regions, i.e Lango, Palwo, Alur, Kumam, Lango and Dhopadhola, are said to be nearly mutually intelligible to Acholi. According to a Peace Corps language manual, Acholi is most closely overlaps with Lango and Alur but even speakers of Palwo, Lango, Dhophadola and Kumam can understand few words in Acholi and vice versa (1).
Ethnologue estimates about 1.5 million total Acholi speakers (2), but these figures are from 2014. I can’t find a more recent and reliable statistic yet.