In Conan there is no advancement beyond level 20 (unless some recent publication introduced Epic levels)
In D20 in general, the extra XP required to advance to the next level is always (current level * 1000).
So the total XP threshold for any given level n is always (SUM n * 1000), or ((n* n-1) / 2) * 1000.
Hope this helps.
However, I advise against playing to Epic levels and allowing unrestricted multiclassing. The idea of the 20 levels is that you get a payoff at very high levels if you stick to one class. If you multiclass, you get a quick benefit but have to give up the late payoff. With epic levels, you don't have to sacrifice anything.
One D20 game I used to participate in had an interesting multiclass rule: 1. you cannot have more than two classes, including PrCs,
2. you must take at least 10 levels in each class you take
3. maximum level is 30 (or 28 or whatever)
This ensured that only focused, single-classed characters could enjoy the Epic bonuses of any class. It was a pretty good system.