Cryptic Caves can be one of the five lands you control. You don't need to control five other lands. It's okay that you'll only control four lands as you finish activating Cryptic Caves's last ability.
If you control two Cryptic Caves and exactly three other lands, you can activate the last ability of only one of the Cryptic Caves. As soon as the cost for that ability is paid, you will no longer control enough lands to activate the ability of the second Cryptic Caves.