If a site collection content database size grow up; can I add a second Content Database to it? if No, what's the solution?
A site collection have a single content database, we can't have 2 content database for a single site collection.
Now couple of things you can do.
Clean Up, delete all unused files, items etc. Clear the recycle bin.
we can also split the site collection into multiple site collections. Like if you have multiple subsites in it than make those subsites a separate site collection n move into different content database.
Create a new site collection, move the old data in that site collection.