Also, keep in mind that advertising at WordPress.com is not allowed. For full details, please see:
If you require advertising, you might be interested in a self-hosted (WordPress.org) installation. There are no restrictions on advertising if you setup a self-hosted WordPress.org blog instead of a WordPress.com blog, but there are some added responsibilities.
Please see this link to learn about the differences:
We have hosting recommendations at the following link if you are interested: