So long as you've either created a faction, or are a moderator in a faction you may claim land from the wilderness, or another faction's land claims--given the faction has a negative faction power to claimed land ratio (ex: 10/5; where the 10 represents their land claims and the 5 represents their faction power). I'm assuming you've made your own faction, and are attempting to claim land. If your power is 15, then yes, you may claim a maximum of 15 chunks. When you die, your power will go down and your claims will become vulnerable to "overclaiming," a term used to describe a faction claiming another faction's land. Therefore, it is wise to build a faction; recruit members, create relationships, but be weary. Don't trust someone right away. There are many players that wish to infiltrate a faction and betray them. +15 power = you may claim 15 chunks for your faction.
15 power means you can claim 15 chunks. A chunk is is 16 x 16 piece of land. You should read @Timmyk14's reply. It is well detailed.
u may have a total of 300 claims if you have a full faction (20 players) because each player gets 15+ pow!