HREFLang XML Site Maps follow the same rules of any other XML site map.  The standards say you have have a maximum of 50,000 URL’s and must be no larger than 10MB (10,485,760 bytes).  As you can imagine mapping XML files to 20 or 30 country alternative variations the file sizes can become quite large.  While you can have individual files this large, our experience suggests you should keep these files smaller at a maximum of 10,000 URL’s per XML site map.   Accordingly we allow you to set the size of your individual XML files.

the "Default" setting is 10.000 URL's per XML site map file

Note:  After each 10,000 a new file is created and is sequentially numbered.