There are a lot of magicians out there, doing their own thing, and through the Kanes' entry into the magical world, we get a few glimpses of them.
At the First Nome in Egypt, magicians walk around on daily business: "Some wore linen robes, some modern clothes. One guy in a business suit walked past with a leopard on a leash, as if that were completely normal" (13.68). For all we know, that kind of thing is normal for magicians.
Young initiates (aged 6-10) also make up the magician population of the First Nome. In addition to being trained in magic, they use scrying (looking for images in a liquid) to pass messages between the various Nomes. It's kind of like using people to transmit telegraphs, we suppose.
When Desjardins tracks Set and the godlings down at the pyramid outside Phoenix, he brings a dozen or so magicians with him. There, they fight demons: "Each magician stood inside a moving protective circle, like a spotlight beam, wading through the enemy with staff and wand glowing. Flames, lightning, and tornados ripped through the demon host. I spotted all kinds of summoned beasts—lions, serpents, sphinxes, and even some hippos charging through the enemy like tanks. Here and there, hieroglyphs glowed in the air, causing explosions and earthquakes that destroyed Set's forces" (37.16).
Problem is, for all their heroics, the magicians are outnumbered—by a lot. This is unfortunately kind of like a metaphor for being a magician in the modern world in the first place: you're outnumbered by forces that are either hostile or don't believe you exist. But if it's either fight or lie down and die, what are you going to do?