In my case, I have one cPanel/WHM Server, let's say W1, and several dedicated mysql servers, let's say D1, D2, D3 ....

Let's say I have a few hundreds of websites host in W1, these websites are web1, web2, web3, ...... web500, ....

How can I do this?
databases for web1 to web 200 will be on D1,
databases for web201 to web400 will be on D2,
databases for web401 to web600 will be on D3, ....

When I add a domain account and create a database for this account, can I tell cPanel which database server to choose from?

Anybody know this?

Thank you,

Nope nothing automatically.

What cPanel will support is a remote mysql server. If you got fancy you could use a something like mysql proxy or mysql cluster to spread everything out.