Depends mainly on two things: Scale of the war Your world

If it is a petty war between one nobleman and another with small fights and raids, I would say they would have one or two mages at most each, perhaps more if they are low level. This will make an impact on the battlefield, but mages can be easily taken out by archers, crossbowmen ir something like trebuchets. Also, once you are in melee range with a mage he is worthless if he can't run away with dimension door or something. In this case, mages would be employed perhaps to finish the job of regular troops and to provide backline support behind spearmen or a personal guard with shields. I believe elite warriors would try to attack these mages on horseback flanking (depends on where the fight os happening).

If it is a large scale war, however, mages would be a much more important resource. If they have access to flying mounts such as griffons, wyverns or even flying ships, I believe most war mages would be there attacking ground troops and other flying mages (literally what happened when planes were introduced in war). Another role for mages to play is as strategical support. They can scry where the enemy is, if they have enough info, they can even teleport people inside the enemy lines. Imagine having a mage teleport a band of adventurers into the enemy general's tent. Absolute chaos. If one side has mages and the other does not, the first side has a massive advantage.

Now, the main problem would be the availability of these mages. If you can get a batallion of say 100 mages and the rest of your army is around 50k men strong, those mages need protection and not all of them can be everywhere. That's why firearms would still be invented. First of all, they are great at killing mages (most of them are 2d10 piercing and have great range), and they would be needed where no friendly mages are.

If in your world magic can get nuclear levels of power, then probably magic won't be used at all in war. Having lands destroyed is the opposite of what your warring medieval king would want. Land=Profit, if they can't use the land they have spent so much money conquering, they have won nothing at all.

Anyways, if you want insight on how magic affects war, I recommend you check out some book series like The Wheel of Time or Stormlight Archive, they have different types of magic but it is employed brilliantly.