OSHA said it best! Buuuut let’s think about this. When Will told Ned Stark about the White Walkers and decided to hear him out only to then realise, “oh shit, Maester Luwin! CALL THE FUCKING BANNERS!!!”
Then 20,000+ Northmen would’ve marched to man the Wall, Cat would’ve spoken to her father and if convinced then the Riverlands would’ve joined.
Eddard would’ve gotten into Robert’s ear about it, let’s just say that he convinced Stannis and the Lannisters to join, along with House Tyrell and Martel to send their Armies North,
If Lord Commander Mormont was convinced of the threat and allowed Mance to bring the Wildlings into Westeros, the warriors staying to defend obviously that’s easily 100,000+ soldiers before the Night King had even properly begun his conquest!
Basically the majority of the Night Kings army consisted of Wildlings, giants, Nights Watchmen, Large Carnivores etc etc.
If the majority of the giants and Wildlings were behind or at the Wall, with no large Night Watch ranging parties going North. The Night King wouldn’t have had anywhere NEAR as many soldiers from the start. The ENTIRE WALL would’ve been manned, leaving the White Walkers with hardly any fuel for their army.
And if we get real technical, King Robert would’ve ordered the Citadel to find out as much as possible about the White Walkers, eventually the Dragon Glass thing would’ve been discovered and consequently, EVERY scrap of Dragon Glass would’ve been mined out of DragonStone and fashioned into weapons.
No doubt the Golden Company would’ve been bought by either the Lannisters or the Tyrells, an additional 20,000+ soldiers. and let’s have fun and say that in their desperation to build an army, they heavily invest in the slave armies in Essos, ie the 8,000 Unsullied! And probably a bunch of other mercenary/sell Sword groups eg. the Second Sons, 2000+. The numbers keep rising.
That leaves the two Targaryens in Essos. At that time Dany might have been married to Khal Drogo but probably wouldn’t have had his support to go to Westeros just yet. So the Targaryen army wouldn’t have existed yet.
But still....If ALL of Westeros was united against the Night King. The war of the Five Kings had never occurred, all those soldiers weren’t killing each other and were now preparing for the Long Night, the Night King having a strong lack of resources and soldiers, and no dragon, probably wouldn’t have been so keen to travel South.
And let’s say he grew some balls and tried his luck. Let’s be generous and say he mustered an army of 10,000-20,000 wights, plus polar bears, a few stray giants.
Yep, have fun Night King! May the odds be ever in your favour....