D’Erlon’s I Corps had about 16,000 infantry at Waterloo.
The figure is an estimate rather than a precise headcount. One commonly cited breakdown gives the corps roughly 16,885 infantry, alongside about 1,500 cavalry and supporting artillery and specialists. That makes 16,000 the appropriate rounded answer for a trivia question asking “about how many.”
D’Erlon’s corps formed the French right wing and attacked Wellington’s left in the afternoon. Four infantry divisions advanced toward the Allied ridge, but the attack was thrown back by a combination of Allied infantry and cavalry. The corps suffered heavily and required time to reorganize.
Different sources produce totals near 19,000 or 20,000 when cavalry, artillery, staff, and other personnel are included. That explains why larger figures sometimes appear. The question asks specifically about infantry, so the rounded 16,000 figure is the relevant one.