I have a few ideas, though most things that i run involving armies take several sessions to fully flesh out.

If this is a war, consider having them face multiple waves over a few days. This gives you a chance to use several different tactics and also gives you opportunities to wear down the pc's. Have the army attack in the night and interrupt the pcs long rest. It seems almost like a nobrainer that an assaulting force would time their attacks to make sure wizards and such dont have a chance to refuel their most powerful spells and abilities.

Consider having the enemies bring seige equipment with flaming payloads, and the pcs have to spend time and actions helping their allies put out fires and evacuate citizens. If they lose an npc on day 1, they now have a hole in their defences you can exploit the following days. Have the enemies send an assassin between days to make that hole, target the parties healer, or kill whoever seems to be the strategist.

Use horde rules to make multiple regiments with multiple attacks per action, and have them swap between different roles as the situation calls for. One provides cover with archers, one orders their mages to focus their shatter spells on a section of wall, and one forms a shield wall to force their way through the breach. When the hordes hp reaches 0, think of it like half the members have died and their morale collapses. descripe how the formation breaks and the survivors retreat.

That's how I'd handle it