I upgraded a system from 20 up to the current release. It was an interesting experience as the upgrade process has definitely improved over the years
It felt like about 5 distinct upgrades, as you do each upgrade, then fully update the system, then prepare for the next upgrade
If you get through the first upgrade you'll probably be fine, but make sure you have backups - the upgrades enforce backups throughout, but I would always keep a full backup before you start