Concerning carry weight, I like to carry one each of both a two handed weapon and a one handed weapon, together with enough ammo to use both without worrying about running out. Unless I take One Hander, then I take two pistols, one for longer range and the other for better damage against the toughest enemies. The one handed weapon is usually a pistol for ranged builds or an Unarmed weapon for melee builds. For the ranged builds, the two handed weapon is usually the stronger weapon of choice for use against the tougher enemies while the one handed weapon is used mainly for ammo conservation against weaker opposition. This is especially useful if using burst fire only weapons such as Miniguns or Gatling Lasers.

For example, for my Sniper (not using One Hander) in the first game, I use the Turbo Plasma Rifle and the .223 Pistol as my endgame weapons. Since the Turbo Plasma Rifle is the strongest single shot rifle, and also due to carry weight concerns, I don't think there's any point to carrying the Sniper Rifle despite its longer range and using the same ammo as the .223 Pistol.

Though in Fallout 2, I have written down for my notes for some builds that I should have a least one weapon that can deal reasonable damage to Frank Horrigan, including one of the following depending on whether I go ranged (with aimed shots or not) or melee: Pulse Rifle, Alien Blaster, Super Cattle Prod, Bozar, Vindicator Minigun. A Pyromaniac build would need to probably use the Alien Blaster against Frank Horrigan due to his armor being able to resist fire damage. Same with a Burst Fire build using the Gatling Laser, as it usually uses the Gauss Pistol for it's range, damage, ammo conservation purpose, and its low AP cost, it would also need to use the Alien Blaster against Horrigan. I know, because I've done this in the past and found that although the Gatling Laser could rip through enemy defenses with Sniper and 10 Luck, it couldn't inflict close to 100 points of damage against Frankie.

In the second game, my snipers use either Gauss weapon (depending on whether One Hander was taken or not) together with the Alien Blaster. The Alien Blaster deals most damage than the Pulse Pistol despite the latter's Weapon Penetrate perk. The Gauss Rifle is better for long range combat and aimed shots than the Pulse Rifle due to the latter's lack of perks that increase accuracy, unlike most other rifles. The Pulse Rifle would be better in the hands of Marcus or a Fast Shot character using single shot weapons. So the Alien Blaster is the pistol of choice against enemies such as Enclave soldiers, Super Mutants, Robots, and Frank Horrigan.

As I said above, I don't think there is a point to carrying a weapon in a similar class if another weapon deals more damage, even if the former has better range. Such as the Sniper Rifle being replaced by the Turbo Plasma Rifle, or the Pulse Pistol being replaced by the Alien Blaster.

How many weapons do you include in your endgame arsenal concerning carry weight?

To be honest, and from my experience. You need to carry solely weapons with energy ammunition, as that is the only kind of ammo you will have access. And in particular for the second game, you need to be able to bypass as much as a DT as possible as most of your enemies will carry some of the best armor.

For the first game, if you go solo, with good social or stealth skill, not weapons are required.

So, for your final answer. I normally only carry a single weapon that fit best to the build I'm trying there. And grenades.

But what if you are built primarily around the usage of weapons that fire off many rounds in one attack, such as Miniguns or Gatling Lasers? It would be a waste of ammo to use it on weaker enemies. That's what you would want a single shot weapon for. Also, if your primary weapon is a two handed one, what do you do if you get an arm crippled? Since you can't use the rifle, you'll need to use either a pistol or an unarmed weapon depending on whether you are ranged or melee?

In the first game is a good concern. But... I tend to go stealth or pacifist on that one (or with many meat shields). For the second game, if I remember well, you will constantly get ammo for the gattling gun from the weaks one. And also, killing them in a single and bloody mess is always nice. GRENADES xD And then pick a secondary weapon from the dead,