Typically birdshot is used on clays and steel plates, buckshot on paper and some steel, and slugs on paper or steel (range permitting)
Buckshot, pellet count depends on what the match director stipulates, but its typically 9 pellet 00buck for 12 gauge and I think 20 something for 20gauge??
9 pellet is typically the max due to time it takes to patch paper targets and destruction of targets if people are shooting 12/15 pellet rounds.
As for the birdshot, you'll do fine with 7.5 target loads, but some like to run heavier shot and more potent loads, especially if their gun doesn't have chokes.
sometimes knocking down heavy steel at distance is difficult with cylinder choke and a target load.
In IPSC shotgun there are no combined ammo stages meaning it will be slug only, buck only or bird only.
There should be more shotgun matches this year in Ontario than in years past.