You'd made another post about Tableau pricing and licensing and in that post I'd suggested that you reach out to Tableau sales. That still holds, you'll be able to describe your needs Tableau sales and they can come to a better recommendation than we likely can.
I can say that a core license starts making sense when you have a lot more than 20 users, or plan to grow to a lot more than 20 users.
It really depends on what are your requirements and what are you trying to accomplish.
Will userbase grow rapidly?
Will dashboard be published on your company's website and other users may view it?
If you will require guest access.
You can talk to your account manager and ask them to give you the trial key of both version and you can install and check out.
If you will have only 20 users, each user cost $420 per year. 20 users cost will be $8400 (You can have any number of cores on your server as far as I know).
Enterprise core license cost more than $300,000 for 8 core and you have to pay 30,000 per year for maintenance so you can upgrade to new versions.
Talk to your sales manager and see which will be the best licensing option for your company, do more research and find out the limit of each licensing type.
Let say if I go with user based licence for 20 users.
If in future users may increase to 30 .then how can I add them to already existing server.
So, They can give you an updated license key and you can just deactivate old License key and add an updated license key or they can modify your existing License so all you need to do is refresh and you will be good to go.