Id like to know which is the best 
practice to upgrade CA SSO from 12.52 to 12.8 in a complex scenario 
with multiple policy server ? 

Release: MSPSSO99000-12.8-Single Sign-On-for Business Users-MSP
Component:

At first glance, as my colleague mention, you might consider a 
parallel upgrade, as it permits to upgrade most of the components 
progressively. 

But in order to study your current architecture and in order to insure 
you choose the best way, we strongly suggest you to get in touch with 
CA Services. 

CA Services 
https://www.ca.com/us/services-support/ca-services.html?intcmp=headernav 

Contact CA Services 
https://www.ca.com/us/contact/services.html 

You'll find the upgrade documentation here : 

https://docops.ca.com/ca-single-sign-on/12-8/en/upgrading 
https://docops.ca.com/ca-single-sign-on/12-8/en/upgrading/parallel-upgrade 
https://docops.ca.com/ca-single-sign-on/12-8/en/upgrading/in-place-upgrade 
https://docops.ca.com/ca-single-sign-on/12-8/en/upgrading/rolling-upgrade