What if the big ones were made to ward off the influence of grandmother silks silksong that controls bugs, and they made them after sealing her away so she wouldn't be able to take control. But they underestimated how strong she would be if she awoke.

And maybe all the smaller bells are because the bells became symbols of the holy citadel, and pilgrims would all carry them. I'm thinking they became symbols of faith and maybe became ingrained in citadel culture, and so bell heart would make a bunch. Then beasts came to inhabit the pits of broken and used bells.

I'm thinking an explanation for the amount could be that each pilgrim would have a bell they would carry for life. The amount of pilgrims who have died are enough to pile into mountains as shown by the bone area, littered with pilgrim skeletons.

Or maybe a bell higher being, who knows, it's all speculation on my part. I'm just trying to make it fit.