Each organization is different. One organization may only require 6 while another may require 20. However for a specific event, the first organization might give you credit for 0.4 units where the second gives you 1.5 units.
Personally, I am ACSM Certified.
The ACSM requires 45 CECs every three years.
They will take ACE CECs; but ACE, for example will give you 1.5 units, and the ACSM equivalent is 15 units. Some use whole numbers, others use decimals. But as Natalie said, they generally represent the number of hours.
It can all be very confusing.
Some organizations recognize things others don’t. They will each give different credits for the same things.
When it’s all said and done, it will all come out the same in terms of investment in yourself. They all want you to continue educating yourself with current practices. Depending on what events you chose, it will typically end up costing you $1,000-2,000 every 2-3 years.