[Pikachu_Dad]

Curious - the website claims different numbers
UNION. 28,450
CONFED. 32,230

The map claims McDowell has 35,000.
- so where are the missing 6,550 Union

The 20,000 + 9,000 +3,000 = 32,000 Confederate troops match

[BroJoeK]

Apparently only 18,000 were actually engaged on each side.
All the rest never made it to the battle, most notably Union Gen. Patterson's 18,000 which could have turned the battle.

So...
If we count everybody within the battle's general vicinity, then:

• Union (including Patterson's 18,000): ~54,000
• Confederate: ~34,000