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Yes, Active Directory requires CALs. And CAL model depends on you. There are 2 types of CALs Per Server and Per Seat
As Per Server is for small companies with one or two servers (each server requires the same amount of CALs determined for concurrent connections), you need to use the second one, Per Seat. Per Seat is also divided into 2 parts: Per User or Per Computer.
In this case you need to choose which one is more appropriate for your needs. Per User means that each user in a domain requires CAL. Then it doesn't matter to which PC/server he/she is logging. Per Computer is used for a server/PC. Then it doesn't matter how many users log on to it.
Per Computer is mostly used when computers are shared between few users (shift work) then you can buy 1 computer CAL instead of 3 user CALs (when 3 shifted work)
Per User is mostly used when you have more computers than users. Then user has CAL and can log on to any of those machines.
When your environment contains similar amount of users/computers then it's up to you what you choose. But then it's better to think how it would look like in the future and the decide to buy licences or simply transform them (as it is free for the first time)
Microsoft allows transforming CALs licensing from User to Computer or Computer to User once for free. Another change costs some money.
As Per Server is for small companies with one or two servers (each server requires the same amount of CALs determined for concurrent connections), you need to use the second one, Per Seat. Per Seat is also divided into 2 parts: Per User or Per Computer.
In this case you need to choose which one is more appropriate for your needs. Per User means that each user in a domain requires CAL. Then it doesn't matter to which PC/server he/she is logging. Per Computer is used for a server/PC. Then it doesn't matter how many users log on to it.
Per Computer is mostly used when computers are shared between few users (shift work) then you can buy 1 computer CAL instead of 3 user CALs (when 3 shifted work)
Per User is mostly used when you have more computers than users. Then user has CAL and can log on to any of those machines.
When your environment contains similar amount of users/computers then it's up to you what you choose. But then it's better to think how it would look like in the future and the decide to buy licences or simply transform them (as it is free for the first time)
Microsoft allows transforming CALs licensing from User to Computer or Computer to User once for free. Another change costs some money.