Hi Dina-
You can use your BoldGrid Connect Key on unlimited sites within the same business. For example, you may use it on a Development, Staging, and Production site. Keep in mind that your Key can be used to purchase BoldGrid Connect Search images, so you should not share your key with sites that you are delivering to a client or another business.
For web design agencies, usually what I recommend is to develop your client’s sites using your premium key, and switch them to a free key when you deliver the site to your client. Everything you’ve created using your Premium Key will remain, and your clients can upgrade if they decide they’d like the premium features.