I have been using the Vang shell cards for years. They come in 6 shell versions so I normally have 6 on the gun.
Shotgun shells are heavy and engagements where shotguns are used tend to be brief affairs with few rounds fired. For typical use I don't see much of a need for more than 6. I'd be plenty happy with 4.
If you have a need for different types of ammo, adding a different type to a butt cuff in addition to the side saddle might make sense.
Slug selects tend to be difficult to find in the real world...but I think there's a question of chicken or egg there. In a lot of instances I'm sure that performing a slug select isn't practical in the rapidly changing environment that tends to define an exchange of gunfire. But in others when it is possible, I don't think it happens because shotgun manipulations seem to be poorly trained, poorly practiced, and poorly understood. Even at an instructor certification level, the slug select drill seems to be one that flummoxes more than just about anything else.
I don't find it particularly difficult or enormously time consuming...but I can also load, fire, and cycle an 870 with one hand. So doing a slug select seems easy by comparison.
If one wants to pursue proficiency with a defensive shotgun, slug select is a good skill to have. It's not one I bother wasting time on in intro or even intermediate level classes as there are other things that better serve the interest of the client, IMO.