If you don't exceeed your monthly download limit, you'll be able to have as many shows as you want.
That's a good question. At it's core, a podcast is a creative's playground. Because of this, you may not find the perfect theme, format, length, etc. the first time around. Even if you do, some of your audience members will prefer somethign slightly different.
The ability to quickly create more shows allows you to experiment and boost your subscribes and downloads across multiple channels.
Publish unlimited series and episodes to grow your audience the way you choose.