many, offer, dedicate, to steam
烝 centers on rising steam and the idea of offering up or dedicating something, as in a ritual. The sense of 'many' likely extends from the image of abundant steam or a large offering.
烝 combines 丞 (which historically relates to assisting or raising) with 灬 (fire), suggesting steam rising from fire. The exact historical development is uncertain, but the character is associated with steaming and ritual offerings.
The top part 丞 looks like a hand lifting something up, and the bottom 灬 is fire. Picture a hand offering a sacrifice over a fire, with steam rising up — that's the idea of dedicating and steaming.
For ジョウ, imagine a steam-powered show where many offerings are dedicated: show -> ジョウ, and the steam rises from the fire.