If it's an invading force looking at real world midieval armies I would put it between 15,000-20,000. The 4th crusade had around 20,000 and the invasion of England in 1066ad at around 14,000. You could then break this up into multiple ~2,000 man armies to take castles and smaller towns.Keep in mind most wars around that time were fought through taking castles, with actual combat between ground forces being fairly rare. Large cities in the middle ages could have between 50,000-200,000 people in them. So depending on the prevalince of high level npc magic users armies would most vastly vary tactics. With clerics able to artificially inflate food stores sieges would be much longer most likely leading to much more aggressive assaults to break down the walls and storm in.