>Space Marine, Space Wolves, Blood Angels, Dark Angels, Chaos Space Marines, Harlequin, and Eldar codices say, "May replace one weapon with one [Army Relic]," or, "May replace one weapon with one of the following."
>Limit of 1 Relic per model, exceptions made for the limited relics within each codex caveated to not replace weapons.
>Dark Eldar, Necrons, Grey Knights, Skitarii, Cult Mechanicus, Adepta Sororitas, Imperial Knights, Inquisitors, and Orks wargear lists says, "A model may take one of the following."
>Limit 1 per model, barring specific caveats
>Khorne Daemonkin has both limits. "A model may take one of the following..." Or "A model may instead replace one weapon with one of the following..."
>Still a 1 per model limit.
>Chaos Daemons only has a limit of one per army, but the codex employs specific mechanics which make it roughly impossible for a character to end up with more than one.
>Tyranids codex says, "A model may replace any pair of scything talons with one of the following." The limit seems to be the number of pairs of Scything Talons a model has, although at least two items also do not replace scything talons and may be taken on top of others.
>Legion of the Damned only has one relic, total.
>Tempestus Scions, and stand-alone factions like Officio Assassinorum, Deathwatch, Genestealer Cultists, and Renegade Knights don't seem to have relics.
>Tau and Astra Militarum only seems to have a limit of one of each item per army. I see no restrictions per-model restrictions, although some Astra Militarum Heirlooms may only be taken by specific models or replace an item.
Most supplements work the same as base codices. Farsight lets models take from both Sig system lists. If you paid the points, you could have 14 SigSystems on one commander.