I can't really comment on medieval era castles, but I can comment on later fortresses that were in use from the 17th through 19th centuries which are similar in many ways to a castle.

The main part of your question is easiest to answer. I've never heard of anyone being locked out of a fortress or castle (I'll just say fort from here on out) on accident, but that doesn't mean it never happened. If it did, the person may have never told anyone about it if they could help it. Or maybe they did tell people but nobody wrote it down. This is the eternal struggle of historians. Anyway, I have heard of rare instances where people barraciding forts up and then abandoned them by climbing over the walls and beating a hasty escape. This might be done when a fort could no longer be held, but you saw no reason to make it any easier for the enemy to claim it. The enemy might even sit there a while until they realized nobody was inside it.

So why would it be difficult to lock yourself out? First, I've never heard of a fort with a door that could be locked from the outside. That would introduce a point of weakness which could be accessed from the exterior of the fortress. A keyhole (which did not exist, and I'll address next) or some other access point to the interior of the gate or to the locking mechanism would just assist attackers in breaching the door as well as making the door itself weaker. A lock which is accessed from the outside would necessarily have removed an interior portion of the gate, making it weaker. Secondly, should the person with the key lose them, or be captured, or have the key stolen or copied, etc., it would grant relatively simple access to the fort's interior. Certain areas within the fort might be locked behind a key, but the gate would not be. By making access to the fort require human interaction you eliminate the chance of someone lock picking or stealing their way in. Obviously this introduces social engineering, but that could be mitigated in other ways.

Here you can clearly see this gate has no internal locking mechanism. The edges of the gate are flush. Many forts now probably have locking doors, but those would be post-war additions to keep out trespassers.

Secondly, the gates to a fort almost always locked by some simple but effective method from the interior. Keys and traditional locks were not used because sometimes the gate needed to be opened or closed quickly in battle when the key might be lost, or the person with them was dead or missing. Instead, bars or braces would be used to keep a fort gate closed. A bar could be placed across the gates to prevent them from opening which could usually be removed by one or two people. In desperate situations spikes could driven into the ground or in the gate's hinges. In truly desperate situations where you have no escape, you might pile as much rubble or other material as you can in front of the gate. These type of locking mechanisms can only be put in place by someone inside the fortress, so it would be impossible to lock yourself out on accident.

Here is the sally port from Castillo de San Marcos, constructed in the late 17th century. You can see it has a draw bridge but no locking door. What looks like a raised portcullus is actually part of the draw bridge mechanism. Here is the main gate, again, a draw bridge. These kinds of mechanisms could only be operated from inside the castle, so it would be very difficult to lock yourself out.

Here is a gate from an early 19th century fort, and you can see the lack of interior key locking mechanisms. You can also see the hinged bars that would have rotated in place to keep the gate shut and only operated from the interior, as well as a sliding bar towards the bottom. This could be easily and quickly operated by one or two people.

Lastly, if nobody is left inside a fort then they aren't all that hard to get into. A ladder or rope and a little gumption would get you inside pretty quickly. Later fortresses would be even easier to get into with their shorter slanted walls. Some were even earthen mounds that would have been relatively easy to walk up when you aren't being shot at. The difficulty of getting into a fort is when they are guarded by people with guns or bows who can attack you from safety.

As to how many people lived or worked in a fort that depended on its size and what was going on. In times of war forts could bristle with hundreds of men or more, and other times only have a skeleton crew of a few dozen. Fort Sumter had only 85 men for its 135 gun emplacements when it was attacked, but was designed for 650 when completed. Even more would have likely been stationed there with preparation. Fort Wagner a Confederate fort made famous by the film Glory, housed 1,700 Confederate soldiers at its height.

I imagine some of the above would apply to older forts since much of the basics of fortifications remained the same, but would let other people chime in.