Ideally, the base "naked" armor class of a human is AC 8.
Renaissance "full" plate "suit" is AC 18. If the base is AC 8, then the rest of the armor adds up to +10 AC.
Something like:
• Great helm: +2 AC
• Metal torso armor (cuirass, breastplate): +4 AC
• Both legguards together: +1 AC
• Each armguard: +1½ AC
Note. The 5e "bounded accuracy" compresses the value of armor into a smaller space, making the +½ AC bonus useful to represent higher resolution when quantifying the value of different pieces of armor. Round down if there is an extra ½ left over.
So a manica armguard on one arm (typically the offhand) grants a +1½ armguard bonus. A standard helmet grants +1½ AC helmet bonus, for a total +3 AC to the base naked AC.
While a standard helmet grants a +1½ AC helmet bonus, a great helm grants +2 AC, and other headgear might grant +1 AC.
The armguard armor is a shield bonus doesnt stack with other shield bonuses.
A longshield or tower shield doesnt stack with a leg armor bonus either.