A content db supports 5000 site collections , however it can increase upto 10,000. It also depends on the factors , like how big a site collection is.A maximum of 200 gb of content db are supported, however in some cases 4tb of content dbs are also present.
5000 limit is for SharePoint 2010 version and 10000 is for SharePoint 2013, 2016 and 2019 versions