17,508 islands, to be precise. About 6,000 are inhabited. The population density of Indonesia is about 340 per square mile, which is approximately 80th in the world. However, Java, the most populous island, has a density of about 2,400 per square mile, so that is definitely crowded. I will be on the island of Sumatra, in the city of Medan.
Try to get in some skin diving while there. I’ve always wanted to dive in areas where sunken WWII vessels are located. Should be lots there.