Running LrC v 12.5 on my iMac and Lr (cloud-based) 7.0 on my Windows Laptop.

[DdeGannes]

My iMac is mid 2015 and cannot be upgraded beyond macOS Big Sur 11.7.10 and as a result cannot upgrade to LrC 13.

However my Win laptop has LrC 13.0.1 installed but it's not synced to the Creative Cloud so that is not a problem.

 Lr (cloud based) app is upgraded to version to version 7 and my mobile apps iPad and Phone are also  upgraded.

Will I have problems with syncing ?

[Rob_Cullen]

So the iMac is 'out' on its own. It will not be part of any sync process, and any Catalog for LrC on the iMac will be a version prior to LrC v13 and therefore not compatible with the Win Laptopv13 catalog.

Now the Laptop with Lr-Classic v13 can be the system that syncs, and all the Lightroom (Mobile / Cloud) apps on phones and iPads will sync.

Be aware that the Lightroom-Classic catalog v13 on the Laptop, when sync is active, will want to download ALL photo files you add, or have, in the Lightroom Cloud/ecosystem. Disk space could become a problem unless you have added memory or external drives.

[DdeGannes]

Thanks for the info Rob, my IMac is my workstation and my main computer, the LrC Catalog is where my collections are synced to the Creative Cloud. 
I have synced a collection a few days ago and I can view fine on my iPad,
I am not sure if I make edits on my iPad and Windows  or my Win Laptop with the latest version of Lr if the edits will sync to my iMac correctly.