Yes, there is a site option to adjust the maximum milestones available, but it can only be adjusted by Luminate Support. Please Contact Support and reference this article.  Once this option has been enabled, you will need to take an additional step by going into your TeamRaiser event and enabling this option.
  1. Click Fundraising>TeamRaiser
  2. Click Edit on the appropriate TeamRaiser event
  3. Click on step 3 - Select Event Options
  4. Click Edit Advanced Options under Related Actions from the right navigation bar
  5. On option 16 - Maximum Number of Milestones, enter the maximum number of milestones desired
  6. Scroll down and click Save