Tough question to answer. If you specify hardware specs, software configs and pipe to the net size, it would be easier to give you an answer.
That said, WordPress.com servers 500,000,000 (1/2 a Billion) unique users per month viewing billions of pages per month with a bunch of commodity hardware.
So let's start small. Even on shared hosting, the database won't be the bottle neck. It will be the CPU, Ram or bandwidth.
Upgrade to a VPS, like linode.com, where resources are guaranteed. Apache will be your bottleneck before the database becomes a problem. Which is why I use nginx + php-fpm + apc.
Upgrade to a dedicated server, where u get all the resources and it will still be Apache that choke holds you under traffic stress. That's assuming you have a 100mb or greater connection to the internet. Which is why I use <drumroll> nginx + php-fpm + apc.
So, to answer your question, the database is rarely the issue, its the hardware and software you use to handle the traffic. Given 2 identical VPS's and 2 identical dedicated servers, you'll be able to handle a lot more traffic by avoiding Apache and using nginx, php-fpm an opcode cache (like apc) and memcached.
That's how I roll.
If you have to use Apache, figure out your server specs and google for ways to optimize Apache to your hardware specs. Use
wp-super-cache plugin or w3total cache plugin and you'll be fine for a long time. Your database won't be an issue there either.
If and when your db ever gets to be an issue, it's the easiest fix in the world. You move it to a dedicated VPS or server. Problem completely solved. Once your getting millions of unique visitors per day, you replicate your DB to a slave and now you have 2. Replicate again and you have 3. HyperDB and SharDB are 2 plugins that make it simple to do.
Moral of the story: DB is the easiest piece of the pie. WordPress can handle it. AppThemes can handle it.
All you have to do is LAUNCH! :)
Make everything as simple as possible, but not simpler. - Albert Einstein
