A few would be OK, but keep in mind that one of the reasons for these challenges is to give people the experience of what it is like to make a map that has been commissioned by an author or publisher. In some cases they only have a general idea of what they want and are waiting to see what you come up with. In other cases, they will be very specific, they know exactly what they want, they just want you to make it look great. This scenario would fall in the middle of those extremes. The challenge is to make a map that is exactly this land mass with specific locations already marked and the rest is open to creativity. So, short answer is Yes as long as these extra islands don't become the focus of the map.