If you have ammunition, any quiver in anyone’s possession (including backup slots) will get filled.
So long as you have some decent melee defence and at least enough protection against the odd lucky hit, you shouldn’t be too afraid of getting stunned by the initial orc onslaught (at least if many/most of them are fairly lightly armed). The stun on contact doesn’t actually do any damage to armour or hit points (if you have the Dodge perk, it remains intact after being stunned and you still have an extra +10-15 defence that makes a hit reasonably unlikely while you’re stunned, especially since the orcs aren’t that accurate). If the orcs are a mix of lightly/heavily armed ones, you can deliberately make contact with the lighter ones that move first, and then the heavy ones will get stuck behind and can be attacked with crossbows and/or pikes.
One of the best strategies against both orcs and goblins (though not so much the orc parties with lots of heavily armed warriors or warlords) is to have some members with Bags&Belts, Quick Hands, 60+ ranged skill and more than one crossbow. Against orcs they can wait for the advance behind melee fighters and then shoot twice from close range, usually hitting home and doing a lot of damage to hit points, so that the melee fighters then have more modest work to do. Against goblins they can move forward and shoot at the ambushers from close range, usually killing or severely injuring at least one or two of them in the first round, so that then (even if one or two people get poisoned) the goblins won’t have enough firepower to stop your advance. The crossbowmen are especially effective if they have high initiative and the Dodge perk, and repeatedly shooting fresh bows at the start of battle (instead of reloading) means that their initiative (and bonus defence) remains high and that they get to act early. It can be much more difficult, however, if it’s awkward terrain that favours the goblins and they have an overseer or shaman …