Party Limit

Under the SPECIAL system, player characters are limited to a certain number of non-player character (NPC) companions in their group. This restriction is called the Party Limit. The number of NPCs following a character at a given time is usually referred to as the "party count."

Fallout
The number of followers a player character can have in Fallout is not limited, other than by the total number of available followers in the game.

Fallout 2
The party limit in Fallout 2 is a derived statistic based on the character's Charisma SPECIAL stat. The party limit is calculated thus:


 * $$\text{Limit}=\text{Charisma}/2$$

When the charisma stat is an odd number, the result is rounded down. Ergo, a character with CH 9 will only be allowed 4 NPCs. Since SPECIAL imposes a strict cap of 10 on all stats, a PC can have, at most, five "normal" NPC followers.

Exceptions to party limit rules
Some NPCs are immune to the party limit rules based on their scripting. These NPCs are programmed differently from other follower NPCs, and are therefore subject to their own conditional rules, separate from the party limit rules. In these cases, the addition and removal of these NPCs from the party is typically dictated by set triggers rather than dialogue.

Married
If the Chosen One got Married in Modoc, their spouse will be added to the party, overriding the limit and cannot be disbanded by usual way, by death or by divorce only. The spouse is counted as a follower so one cannot recruit new followers above limit.

Laddie
Laddie, the dog from Modoc, is automatically added to the party when the player character undertakes the quest to find Jonny. If the player character is below the party limit, Laddie counts as a follower NPC. However, regardless if they are at the party limit, Laddie still joins the party. The NPC is not detached from the party until death or the quest conditions are fulfilled, and as such cannot be dismissed like another follower.

The Pariah Dog
The special encounter Pariah Dog is another limit-exempt NPC. In this case, the dog is automatically attached to the Chosen One's party whether or not they are at the limit. If the party isn't full, the dog is attached and counts against the party limit in terms of recruiting other NPCs. If party limit is reached, Pariah is still attached to the party and counts as a follower, meaning that unless the dog dies, the player character will not be able to recruit a new follower until the party is back below the limit.

Calculating In Limit-Exempt NPCs
If a character has a Charisma score of four, then that PC can have two NPCs as followers (CH4 / 2 = 2). For the purposes of this illustration, the character has two followers, Marcus and John Cassidy, meaning the party count is two. The party is at its limit. If the player character comes across the Pariah special encounter, the dog is attached to the party. As the Pariah Dog is treated as a follower NPC, the party is now over the limit (3 out of 2), but the rules programmed into Pariah override the party limit and insert the dog into the party. If the PC wanted to recruit, for example, Vic, they'd have to fire both Marcus and Cassidy to get the party below the limit; firing both reduces the party count to 1 (Pariah), enabling the recruitment of a new NPC.

Fallout 3
Aside from glitches and modifications, the Lone Wanderer can only have one permanent non-Dogmeat-type companion, as well as Dogmeat himself, as company. An unlimited number of temporary type companions can be added in addition to Dogmeat and the permanent companion.

Fallout: New Vegas
Overall, the same rules are carried over from Fallout 3, as the Courier can have only one humanoid companion and one "animal" companion. The main difference from Fallout 3 is companions cannot die when playing in Casual mode. Instead, companions can be only rendered unconscious. However, in hardcore mode, normal rules of death apply. There are also a few ways for companions to die in Casual mode, however, as in these examples:

After having Craig Boone for a certain amount of time, and one gains enough "points" with him by either completing some quests for the NCR or killing specific legionaries, one can initiate dialogue (through the companion wheel) and get him to open up about his wife. Once his story is finished he can die. In his inventory (specifically when he's dead) one may find a special item on him.

During the Birds of a Feather quest, delivering Rose of Sharon Cassidy to Jean-Baptiste Cutting, he will kill her, provided not being interfered.

Additionally, any follower, aside from Lily or Raul (they need pure, unmutated flesh), can be sacrificed to Mortimer in the quest Beyond the Beef as a meal for the White Glove Society.

Possessing the alien blaster, or YCS/186, shoot at ED-E until it becomes unconscious and falls on the ground. Shoot the unconscious ED-E several times more with the blaster, and ED-E eventually explodes like in Hardcore mode.

Fallout 4
So far it is almost sure that one companion is allowed at a time in Fallout 4, unlike other games in the Fallout franchise.