This is a statement from the novel :
This Lu Bu is a marvel," said Dong Zhuo. "If he were only on my side, I would defy the whole world!"

What does it mean ? at that time Dong Zhou had double or more soldier than all any landlords had combined. Why did he want to have lu bu ? becos he was worth of thousands soldiers.

second statement :
Luckily, two other troops led by Qiao Mao and Yuan Yi came up and rescued the wounded Wang Kuang, and Lu Bu pulled back. The three, having lost many troops, withdrew ten miles and made a stockade. And before long the remaining five commanders came up and joined them. They held a council and agreed Lu Bu was a hero no one could match.

again, the landlords had thousands of soldiers, and their progress were being held up by Lu Bu alone. From here we can assume that Lu Bu actually had the strength of thousands of soldiers.