Well done. I was waiting for someone to mention the "drill" word. Timely abandon ship drills, if carried out properly, would immediately highlight any shortfall of lifejacket numbers and other safety equipment. Regular drills should be second nature to sailors and marines.
You are right, most ships these days are required to carry at least 100% personal floatation devices by SOLAS regulations, (many carry 200%). However, the RTN is a military service and it will have its own regulations. But as a naval vessel, this ship will have carried an extra marine detachment quite regularly, so provision for extra equipment should have been part of the vessel's own safety plan.
I really hope and pray that they can save some more lives today.