MSFT Support Upto 200GB Content database size (general usage scenarios) and upto 4TB for a Content database size (all usage scenarios). So its depend, how you guys use the SharePoint.
Another Thing to think about is, How many Site Collections(Not Subsite) you have? If only one site collection and approaching to 200GB size then you definitely think about to splitting it to couple of Site collections, also clean up the stuff. A site collection can be the same size as content DB.
Check this link for SharePoint Limit & Boundaries.
Yes you can add the more Content Database into the web Application, but keep in mind, one Site collection will be only in one Content DB(a site collection cannot be in 2 Database). Once you add another content DB( Either from Central Admin or Powershell), you can provision new site collection in that db.
Check the below blog, for the Creating New Content Database and provision site collection in it.