Upgrade? How do we know how pre upgrade how many Transforms will need to be upgraded?

When you open the 2022 workspace in Form 2024 you will see the number of transformers that are upgradeable in the navigator:

Noting as well that Workspace Transformers do not necessarily “need” to be upgraded.

Most Transformer upgrades involve changing the behaviour of the previous Transformer through enhancements or logic that has been rethought, and so your workspace can behave differently after Transformer upgrade.

It is why I am very careful to read the Change log accessible direct from the workspace Transformer Upgrade wizard(s) to determine if this will impact how the workspace works before upgrading the Transformer.  Some workspaces we have still have FME 2018 vintage AttributeManagers, Counters and TopologyBuilders left in place because the later FME Transformers output results that are fundamentally different from what we originally designed the workspace to do.

I agree with @bwn , the number of transformers that needs to be upgraded is most likely zero.

Upgrading transformers without checking all the results before and after very thorougly is a recipe for (subtle) problems.