You may consider switching to the Free plan, which allows for 10 users or fewer. All your data will stay safe. If your instance uses more than 30 GB total, you may decrease the number of users in your subscription as long as the total disk space used by your instance does not exceed 3 GB per user. For example, if your current YouTrack instance with 15 users uses 33 GB of disk space, the minimum number of users you may downgrade to is 11.
Administrators may decrease the number of users in a subscription via the Licensing portal: sign in to your JetBrains Account, and under the Licenses section next to the YouTrack license, use the menu option "Check license allocation and view upgrade options" to downgrade.