Three seasons of The Mandalorian had aired by April 2023.
The first season premiered on Disney+ on November 12, 2019, and concluded in December of that year. Season two followed from October to December 2020. Production and release schedules were affected by the COVID-19 pandemic and by the extensive visual-effects work required for the series, so the third season did not arrive until 2023.
Season three premiered on March 1, 2023, and its eight-episode run concluded on April 19, 2023. Therefore, by the end of April 2023, all three released seasons had aired. Each of the first three seasons contains eight episodes, giving the series 24 episodes at that point.
The number can be confused with planned or discussed future installments. A fourth season entered development, but later franchise plans shifted toward the feature film The Mandalorian and Grogu. That later development does not change the historical answer for April 2023: the released count was three seasons.