Micro.blog creates a free extra yourname-tweets.micro.blog blog by default, and if you want multiple tweet archives you will need to create extra blogs to hold them. Those will be billed at $5/month like a normal blog. Make sure to name them with the format something-tweets.micro.blog. The “-tweets” part is handled in a special way by Micro.blog.
You can create new blogs by clicking Plans → New Blog.