fwm: Most shooters prefer carrying their shotgun rounds on a separate, dedicated belt. This, usually worn just above the main gun belt. In most cases, featuring 10 or more shotshell loops. These will vary a bit in configuration, depending on the individual's shooting style.
Most stage instructions won't require that many shells expended....usually 4-plus is the norm. But...misses can and do happen. So a few spare shells are normally included. In most cases, a shooter can get away with 6-8 shotshells on their belt, or slider.
My shotshell belt features an additional 10-round strip for carrying spare pistol/rifle cartridges. This, immediately to the right of the last loop on the shotshell belt. Within easy reach of my right hand, when performing rifle/pistol reloads on the clock. This is far more cartridges than needed under most circumstances, though. Usually 4-5 is plenty. On either the belt, or a slider.
By the way, pistol reloads aren't seen very often, these days. Most times, it's a rifle reload. But, a shooter should still practice this skill. As pistol reloads do happen, from time to time.
Some shooters will prefer sliders. In most cases, these are easy to set up and access. Oftentimes, a shooter will reverse their belt and holster setup, leaving the buckle in the back. In this way, the entire "front" of the belt is free to hold either shotshell or cartridge sliders. It works.
Or, when wearing a dedicated shotshell belt, a cartridge slider may be attached to the shooter's gun belt.** Usually either to one side or the other of the buckle. Depending on preference. I've also done this in the past. Slider mounted immediately right of the gun belt's buckle. Simply a matter of dropping my right hand down, to access a cartridge. Worked well. Though since then, have come to prefer the cartridges mounted on the shotshell belt.
Some shooters will even wear a cartridge slider immediately behind either holster. Tried this....found it a bit awkward. But, does work for some.
Also, you will see cartridge loops directly attached to a gun holster. A fairly good workaround.
**Note: Shotshell sliders may not be attached to a shotshell belt. This is disallowed, under SASS rules. Refer to the SASS Handbook, for further details.
Some shooters even prefer a bandolier-type of shotshell/cartridge belt. This...slung diagonally across the chest. Not seen all that often but, another alternative.
As you can see, there are a number of options a shooter can follow. It all depends on personal preference.
Hope this helps.
Al
ps: Since your location is SW Ontario. Be sure to check the OSASF website, for upcoming matches. Lots going on.