What is the maximum number of characters a filename can be?

A. This depends on if the file is being created on a FAT or NTFS partition. The maximum filename length on a NTFS partition is 256 characters, and 11 characters on FAT (8 character name, . , 3 character extension). NTFS filenames keep their case, whereas FAT filenames have no concept of case (however the case is ignored when performing a search etc on NTFS). There is the new VFAT which also has 256 character filenames.

Learn more: What is the maximum number of characters in a domain name

NTFS filenames can contain any characters, including spaces, uppercase/lowercase except for the following

" * : / ? < > |

which are reserved for NT, however the file name must start with a letter or number.

VFAT filenames can also contain any characters except for the following

/ : | = ? " ; [ ] , ^

and once again the file name must start with a letter or number.

NTFS and VFAT also creates a 8.3 format file name, see How do LFN's work?