If introtech standard game (not a campaign), 1 ton per weapon is usually enough, because without CASE ammo explosions are a concern. CPLT-C1 has only 8 rounds per LRM-15 but that's all right, given its thin armor, and rather good secondary battery of lasers with heat sinks to use them. The Enforcer is fine with its single ton, i think, just needs some care how to use it all.
The Rifleman 3N is a weird case where you probably don't need nor want more ammo. The 'Mech is thin-shelled, so more ammo means more ammo explosions. The 'Mech runs hot, meaning more ammo is not good. But thanks to these attributes, you'll run out of ammo quickly and so the heat stops being a problem, then you can keep firing the lasers often, it isn't like you're going to be around for a long anyway. Otherwise, if you want a similar ballistic-carrier/anti-air 'Mech, pick the JagerMech maybe, it has sufficient ammo for introductory games.
Obviously there are exceptions, you probably want two tons for AC/20 or otherwise you need to be really careful about the shots you pick, while you really don't want one ton of machine gun ammo in any circumstance (half a ton is way more than enough for 4 machine guns [ie 25 shots per gun] for standard game).
Range does play into this as well, a long range 'Mech is likely comfortable with more ammo, but even so armor is also an issue. Like the Catapult i mentioned above. It is a balancing act between useful longevity and dangerous longevity.
Otherwise, about 15 shots is "good enough". Less is passable, to be balanced against role, range, weapon, and armor. More is good, but again there needs to be some consideration of vulnerability to ammo explosions, a lot of 3050s era 'Mechs for example insist on having two tons for single Streak SRM-2, meaning far, far more ammo than you can ever hope to use, presenting considerable explosion hazard. Fortunately truly over-ammoed 'Mechs are a rarity, and some weapons can't have too much ammo really.
Special ammo is naturally different story. You don't need much more ammo capacity, since you're unlikely to be using special ammo constantly, but some good special munitions halve ammo amounts. Again, a balancing act. I prefer to leave special ammo to specifically chosen specialist 'Mech rather than having excess ammo in normal combat 'Mechs for special ammo.