From the documentation:
Model file names must start with a letter and can contain letters, numbers, and underscores. The file name must not be:
A language keyword (e.g., if, for , end) _A reserved name: 'simulink', 'sl', 'sf' A MATLAB® software command The total number of characters in the model name must not be greater than a certain maximum, usually 63 characters. To find out whether the maximum for your system is greater than 63 characters, use the MATLAB namelengthmax command.
To understand how MATLAB determines which function to call when you specify a model name, see Function Precedence Order (MATLAB).
If I understood your question right, the number you are looking for might be 63. Use namelengthmax() to be sure.