My house breaker box have only breakers which seat only 1 wire. No seat for a load neutral. Can I still install a GFCI Breaker?
For "reasons", the US electrical system normally only has breakers on hot and not neutral. That is why hot goes to breakers while neutral goes to a bar. That is also one of the reasons keeping neutrals matching their hot wires is so important - an overloaded mismatched neutral won't trip a breaker. 

GFCI breakers need to monitor neutral in order to detect ground faults. That is why GFCI breakers have a neutral connection as well as the traditional hot connection. If you don't connect the neutral to a GFCI breaker then even a small load will register as a ground fault. The result is that when you install a GFCI breaker to replace a regular breaker you move the neutral wire from the bar to the breaker, resulting in the configuration you've seen elsewhere but not (yet) in your panel.

Most breakers will have a one-to-one match between hot and neutral - i.e., one white (or gray, but far more commonly white) neutral wire for each colored (most commonly black, but can be red or blue or any other color except green, white or gray) hot wire. However, there are some legitimate exceptions:


MultiWire Branch Circuits (MWBC) - These will have two hots and one neutral. The two hots need to be on opposite legs (i.e., the voltage from hot to hot will be ~ 240V while the voltage from each hot to neutral will be ~ 120V) and breakered so that they are turned On/Off together. An MWBC provides two 120V circuits that share a neutral.
240V circuits with Neutral - These will have two hots and one neutral, similar to MWBC. These are used typically for ovens &amp; dryers which need 120V power for controls/lights and 240V for heat.
240V circuits without Neutral - These will have two hots and no neutral. These are used typically for water heaters and similar large loads that need lots of power but don't have any 120V components.


The end result is that the number of neutral wires will typically be a little lower than the number of hot wires.

In a main panel (but not a subpanel) you can also legitimately have ground wires (green or bare) connected to the neutral bar. It is debatable whether that is "good" or not, but it is legal as the ground &amp; neutral are bonded together in the main panel.

Finding the matching neutral for each hot can be tricky. With cables it is usually not too hard as everything in a single cable has to be part of the same circuit. But with conduit it can get tricky as multiple circuits can share a single conduit.