What Is Microsoft Recommended RAM for Windows Running SQL Server Databases

Here are the details of my server configuration to help with suggestions.

Can someone point me to a white paper by Microsoft that at least give recommendation on RAM allocation for windows running SQL Server databases based on db sizes running on the system?  

•      Windows Server 2008 R2, Service Pack 1
•      SQL Server 2012
•      Server RAM: 24GB
•      CPU: 4 processors
•      System Type: 64-bit operating system
•      Storage Controllers: LSI Adapter, SAS 3000 series, 8-port with 1068
•      Microsoft iSCSI initiator
•      Disk drives: VMware Virtual disk SCSI Disk Device

These are the numbers of users’ databases currently online on the server and their sizes. All dbs are OLTP.
•      Database 1: 100.75mb
•      Database 2: 124.780MB
•      Database 3: 7.25MB
•      Database 4: 11.00MB
•      Database 5: 100.00MB
Excluding the systems databases.

Jobs are scheduled to rebuild indexes every 2nd weekend of the month. Some of the dynamic tables have their FILLFactor set and we update statistics every night on some tables.

I just needed to know instead of requesting for more memory for SQL Server all the time, just have a document as a guide. So, even when configuring a new sql box you know exactly by looking at the db sizes what size of RAM to request.