Yes you can
On a Designer plan, you can have up to 10 active websites and on an Agency plan you can have up to 25 active websites at any given time.
Once you have finished building a website you can move it from your list of ‘active' websites to your list of 'inactive' websites and free up a spot to add a new project to your list. When you do this all SquareKicker design changes will remain live on the website (even if your SquareKicker plan expires). If you wish to make changes at a later date you can add the site back into your list of 'active websites'.
Click here for a full description of how the 'active' and 'inactive' lists work.