A "WAD" (standing for Where is All the Data?) is a data package format was originally developed for and used in Doom. It functions in the same way a modern PAK file operates, acting as a single mass of game data and providing minimal lump compression where appropriate. Inside a .WAD file, data entries are refered to as lumps.
A important restriction inside the WAD package is the eight character entry name length for all lumps. When importing lumps into a .WAD, the filename extension is lost (this is because it is not needed - Doom is programmed to "know" the names of special lumps it needs to operate properly).
Other restictions include a maximum height of 128 pixels and 32768 pixels in width on Patch entries and Texture subentries.
Being a 16-bit format. It is reasonable to assume that the maximum number of lumps that can exist in a WAD is 65536 (assuming a entry can exist on index number 0).