Yet another question about replacing a 15 amp breaker with a 20 - 12ga wire
That situation is beyond belief.  To put a 15A breaker on one 12AWG circuit is one thing.  To do it for all of them, doesn't make sense, nobody does that.  Occam's Razor says the likeliest explanation is you have aluminum or copper-clad-aluminum wire.  Look at the cable jacket markings and get back with us. 

Putting 10 receptacles on 1 circuit and 2 on another makes perfect sense if it's appropriate for the loads.  10 might be bedrooms, 2 could be kitchen, bathroom or y'know, a home office. At the end of the day, you know what your loads are and feel free to rearrange.  

The home office and dehumidifier on the same circuit... That is simply a case of someone overloading a circuit. Who did that... The cat?  Upsizing breakers won't save you from overloading circuits if you're not conscious about what loads you are asking a circuit to bear.  It's worth your time to identify which receptacles are on that circuit, total up the nameplate rating of their loads, and make sure the number is sane.  And if necessary rearrange some circuits.  Perhaps the dehumidifier receptacle can be moved to another circuit.  

Panel being full is not a disaster, and it doesn't require the panel be replaced.  It is not that hard to fit a subpanel right next to the main panel, and move circuits there, especially if that area of the basement is unimproved.  My suggestion is take the lesson and get a huge subpanel, 42 spaces is not excessive.  

Partly because you need full spaces for AFCI breakers, which are the quickest and easiest cure for aluminum wiring's various worries.