Normally, there are four to ten per level. One of them is chosen at random each time a player joins or respawns. A respawning player will telefrag any player or monster who happens to be standing on that deathmatch start.
Multiplayer mode allows up to four players, so a level needs at least four deathmatch starts in order to be used for deathmatch games; the engine will exit with an error message if there are less than four. MAP28: The Spirit World, for example, had only two (in version 1.666 and earlier), and therefore could not be played in deathmatch mode.
The maximum number of usable deathmatch starts in vanilla Doom is ten. If a map contains more than ten deathmatch starts, only those with the ten lowest Thing numbers are used.