These crossbows and firearms can be shot 5-8 times (see each weapon's description), but then you need to spend 3 actions to fully reload them. The shootist bandolier can be used to reduce the repeating hand crossbow's reloading time to 2 actions.
These firearms have two barrels that are reloaded and shot independently, which basically allows you to shoot twice before reloading. The breech ejectors are consumables that make you reload both barrels at once, allowing you to shoot twice again, for 10gp.
You can have several loaded crossbows/firearms on your person, but paying for more than one or two fully-runed weapons is prohibitively expensive outside of the automatic bonus progression variant rule. Blazons of shared power make this more affordable, but only if you dual wield two predetermined weapons.
Even then, you're generally just trading a reload action for a draw action, so to benefit at all you would need to avoid the draw action (start the encounter with both weapons drawn, quick draw or similar feat, gloves of storing, retrieval prism, or thousand-blade thesis) and/or you need to draw a weapon that can shoot more than once before reloading (repeating or double barrel).
What's more, if you don't have a hand free because you are dual wielding, then you can't reload. Capacity weapons allow you to bypass this constraint, while a pair of repeating weapons shouldn't need to reload mid-encounter. However, the other weapons need a feat like dual-weapon reload, which would be incompatible with other, better, reloading feats that actually improve your action economy.
Most of these actions let you make a melee strike or use a skill action (depending on your way) and reload as an action. Gunslingers gain one at 1st level, but they are also available at 10th level via the Gunslinger archetype's practiced reloads feat.
This feat lets you reload and shoot a firearm as an action, but it has the flourish trait and causes misfires on a failure. It's available at 2nd level to Gunslingers, or at 4th level via the Gunslinger and Unexpected Sharpshooter archetypes.
This works especially well for Investigators and Tome Thaumaturges because they can predict the failures.
This feat lets you stride, step, or sneak and reload as an action. It's available at 4th level to Gunslingers and Rangers, at 6th level via the Alkenstar Agent and Archer archetypes, or at 8th level via the Gunslinger, Ranger, Game Hunter, and Drow Shootist archetypes.
This feat lets you reload and shoot a hand crossbows an an action, with none of the risky reload drawbacks. It also kinda works with repeating hand crossbows. It's available at 6th level via the Drow Shootist archetype.
This feat allows you to reload or strike with your innovation as a free action, though this has the unstable trait. It's available at 6th level for Inventors, or at 12th level via the Inventor archetype.
Allows you to reload or stand as a reaction when you're critically hit. It also gives you some resistance to the triggering damage and a bonus for when you shoot back at your aggressor. It's available at 8th level via the Shieldmarshal archetype.
This feat lets you shoot and reload a simple firearm as an action, but it has the flourish trait. It's available at 12th level via the Bullet Dancer archetype.
Other unavailable feats
The gunslinger also has other feats like dance of thunder, and perfect readiness, but they are over 10th level and unavailable via archetypes.
As explained in this answer by Ifusaso, there is no rule preventing a familiar from reloading your weapon even while you wield it, and if so, then a familiar (especially one with the independent ability) can easily save you some actions. However, Mark Seifter (a Paizo designer) has claimed that your familiar needs to hold the weapon to reload, and in that case the action economy really doesn't work in your favor. In short, ask your DM.