we have here 10 work stations clients and every workstation client is having access to every server (lets say 5 Windows servers with different version). How many CALs requires

I think You need 10 device Cals server 2016.to license 10 workstations
This will cover the older. Might as well get 2019 Cals device based licensing.

My understanding is you'll only need 1 cal per PC or per user. That covers either the user accessing any windows server or the workstation accessing the servers. So you'll need to decide what is it you've got more of, users, or devices. Odds are you've got more devices than users that actually touch the servers so i usually suggest user CALs. This is because even though you may have 10 workstations and 10 users, you'll also need to count printers, and other devices that may get a DHCP lease from a windows server if you go with device CALs.

Now if you end up having a lot of users who rotate their shifts and share computers than device CALs are a better deal.

The problem with CALs and Microsoft in general is usually they don't even understand the rules and often provide mixed answers when asked. To me user CALs are the simplest answer in most work environments.

cals are different levels... lets  if only file server access then u need only 10 cal for 10 machines is enough

lets say in those servers you have SQL then u have to take SQL cals

if includes Exchange then you have to get Exchange Cals,

like the same way, all the applications i.e RDP, SCOM Etc. need separate cals along with windows cals

all the best

Commonly, there would be more users than workstations.