You have the challenge of the tandems.
(Which is a Google-nope, but won't be once Google indexes this page. Yay!)
Two big things have happened in the world of residential electricity in the past few decades. (Well, actually a lot more things have happened. But these two are relevant here.)
New homes require a lot more circuits than they used to. While old homes, generally, can get by with what they were built with - i.e., code doesn't require most changes to be retrofitted to existing houses - homes are often expanded, renovated or changed in ways that require new circuits. Some examples:
- Full kitchen renovation - need to bring up to current code with at least two countertop receptacle circuits. Circuits often added for microwave ovens, dishwashers and disposals which are more common then they used to be.
- Bathroom renovation - if additional receptacles are added and the only receptacles are on a shared circuit then you may need to add a new bathroom-only circuit. Circuits are often added for exhaust fans, heaters, towel warmers and other appliances.
- Electric vehicle charging - didn't exist until a few years ago
- Basement converted to a workshop - you can never have too many circuits in a good workshop!
- HVAC upgrades
and many more. As a result, a 20 space panel (or even smaller in some older homes) can easily run out of space. Sometimes, especially if the old panel has fuses, the solution is to replace the panel with a larger one. Sometimes you can add a subpanel. But if the panel is in good shape and has tandem (a.k.a., twin or double-stuff) breakers available for it then that is often a great solution. Tandems don't work for everything, but where they work they can often avoid, or at least significantly delay, a panel replacement.
AFCI and GFCI
Arc-Fault Circuit Interrupters (AFCI) primarily protect against fires. They watch for telltale electrical noise that is caused by arcing. AFCIs started with bedrooms (because of concerns about similar problems with electric blankets) and gradually rolled out to most other rooms in the house except those that require GFCIs, though I think some rooms may now require both AFCI and GFCI.
Due to the way AFCI works, it is best done either together with the breaker (I call this AFCI/breaker) or as close to the panel as possible. I believe that where code requires AFCI, it normally has to be either AFCI/breaker or installed (a) at or before the first outlet (i.e., not: breaker, receptacle 1, AFCI, receptacle 2; even if the current code only requires AFCI because of the location of receptacle 2) and (b) with wires in metal conduit between the panel and the AFCI. That effectively means that unless you already use metal conduit anyway that the place for AFCI is either as AFCI/breaker or in a box next to the panel connected by a conduit nipple.
Ground-Fault Circuit Interrupters (GFCI) are primarily about life safety. They detect an imbalance between hot and neutral on a 120V circuit (most things), between the two hots on a 240V circuit (e.g., water heater), and between the two hots and neutral on a 120V/240V circuit (e.g., MWBC, some HVAC). From a safety standpoint, GFCI can be anywhere up to and including the point of use. However, GFCI/receptacles are currently (at least generally from what I have seen) only available for 120V receptacles. As a result, if you need to protect an MWBC or other 240V or 120V/240V circuits then you need to use a GFCI/breaker.
The Tandem + GFCI/AFCI problem
The problem is that almost all GFCI/breakers and AFCI/breakers are only available as full-size breakers. There are some Siemens tandem AFCI/breakers. I expect that other manufacturers will bring out tandem AFCI and/or GFCI breakers over time. But they aren't out there yet.
As a result, generally speaking if you have tandems in your panel then you can't swap them out for AFCI or GFCI. But all is not lost if you really want to add this protection.
GFCI can be installed as receptacles - familiar to most of us in kitchens and bathrooms - and also as deadfronts. A deadfront has no receptacle on it but has the TEST and RESET buttons and some indicator lights and can, as can a GFCI/receptacle, be wired up to protect other receptacles down the line. This will not work for MWBC or other 240V or 120V/240V circuits. The protection here is equal to that of a GFCI/breaker.
AFCI can be installed as receptacles or deadfronts. However, it must be wired up near the panel, as discussed above, in order to provide complete protection. This will not work for MWBC or other 240V or 120V/240V circuits. The protection here is equal to that of a GFCI/breaker.
And finally we get to the ground issue. Despite the name, a GFCI does not actually need a ground wire, and neither does an AFCI. Due to the way that a GFCI works, it can effectively provide protection equivalent to having a ground wire. How? If current would have gone to ground due to a problem, and there is no ground wire, the current won't go anywhere. No current flowing, no problem. But if that current finds a path to ground through a person then the current flow will no longer be balanced between hot and neutral, and the GFCI trips. Problem solved! As a result, using GFCI with a sticker that says "No equipment ground" is a code-compliant way to solve the ground problem. If the circuit requires (in a new installation at least) GFCI anyway then this definitely makes sense. But even if the circuit would not otherwise require GFCI protection, it can make sense because it allows you to install additional 3-prong receptacles on the load side of the GFCI, even though they don't have a ground wire, as long as you also put on the stickers.
As far as I know, installing AFCI (whether AFCI/breaker, AFCI/deadfront or AFCI/receptacle) does not provide "instead of a ground wire" protection. As I understand it, some AFCI/breakers provide limited GFCI functionality, but that is not at a level that really substitutes for a ground wire in the way that GFCI does.