Operating System Version? Filemaker version? What application is at the front when error message appears? What are you doing when error message appears?
Where are the databases located? Local on the computer? On a server or remote location?
How long has FMP been used? Has the error message ever appeared before? What changed recently?
More information is often useful.
OS X 10.9.1, FM Advanced 12.0v5, What application is at the front when error message appears? don't understand. When the error occurs I am going to a different layout. If there is data in the layout I get the message. When there is no data the message doesn't show up. The database is local on the computer. FMP been used since FM11 so 1-1/2 years? The error has occurred before. I have a table with 61 fields and 148 records and the error seems to be related to this table. I just added a field today to the same table before I got the error. I just deleted the field but still got the same message. In the past I went into the table and clicked modify and removed some of the fields from the table view. Didn't seem to help me this time.
I used to get this error all the time back when using an FMP 5.5 database. Back then, you were limited to one table to each file and easily had a great many different FIleMaker Files to a single solution. Since FileMaker can have no more than 50 files open at a given time, it was very easy to hit this limit back then and I had to script some navigation buttons to close one set of files before opening a new set or before accessing a layout that contained references to files not currently open--which is what I suspect is happening here.
You'll need to look at the design of your database system. Is it designed to be one table to a file? Perhaps one interface file and then one data file for each table? If so, this error message is not all that surprising. You'll either need to combine some tables into a single file (a fairly simple change if you have a split data/interface system in many cases) or script a process that closes files not currently in use as you click buttons or otherwise perform scripts to move from one part of your system to another so that you don't go over this limit.