How do I tone down the flavor of garlic in spaghetti sauce
The punchy, raw flavor of garlic is very volatile, and it'll change as the sauce cooks. Garlic (like all alliums) gets a lot more mellow as it cooks, and the extremely intense flavor you're tasting now will fade into the background.

So you simply may not need to do much now - taste the sauce again in a couple hours and re-evaluate. There's not a lot that you can do to target the garlic flavor specifically, other than adding more of the other ingredients to restore the overall balance of the sauce. If your overall cooking time is, say, 6 hours, and you check in another 2, that still leaves you a decent amount of time to add more of everything else for the last few hours of cooking.

But don't panic immediately; give it a little time and see how well it mellows out. It's likely fine and you have ample time to make more corrections.