Dumb q but

how many insignias/runes can a piece of armor contain?

Each piece can hold one insignia and one rune; the insignia being the prefix and the rune being the suffix.

E.g.

Survivor's Dragon Helm of Minor Swordsmanship.

Insignia ----> armour piece's name ----> rune.

I am not really sure,but I think it is limited,some runes will cancel out others.That is why there are stacking and non-stacking runes.Sorry I can't be of much help,I am still learning about it myself.I am sure others will know more,there are a lot of really helpful people here on the forums.
S_S

Stacking has to do with two buffs of the same attribute. Your headpiece comes with a +1 whatever attribute. And it is stackable, which means you can add a rune to the headpiece to increase your attribute buff by that additional amount.

Survivor's -- Dragon Helm (+1 Swordsmanship) -- of Minor Swordsmanship
gives a total of +2 to your Swordsmanship attribute

Survivor's -- Dragon Helm (+1 Swordsmanship) -- of Major Swordsmanship
gives a total of +3 to your Swordsmanship attribute

Survivor's -- Dragon Helm (+1 Swordsmanship) -- of Superior Swordsmanship
gives a total of +4 to your Swordsmanship attribute

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Non-Stackable means that if you have more than one buff of that attribute only the largest one will be counted.

Survivor's Kurzick Cuirass of Superior Axe Mastery means you have +3 to your Axe Mastery attribute.

If you also have
Survivor's Kurzick Gauntlets of Minor Axe Mastery you will still only have a +3 atrribute buff.

Yeah the only stacking runes are "Attunement" and "Vitae" runes. That means you only ever need one of the other types of runes.

Hope that helps