Sharepoint server

use minimum required configuration mentioned with product online documentation
it will suffice your requirement as connecting user base is 20 only

To answer your questions in bit detail:
If U are looking for SharePoint 2016, you need more resources though you have only 20 users as microsoft have raised minimum hardware requirements
U need Full version of SQL software additionally, SQL Express is not supported
U need at least single server with 16 GB memory with 4 cores with two drives (80 GB system drive and 100 GB data drive), this instance will have minimum recommend services to run basic SharePoint and you need to install SQL also on same box
alternatively you can use server with 24 GB memory which gives you all available services
Better start with 16 GB and increase memory if required
https://technet.microsoft.com/en-us/library/cc262485(v=office.16).aspx

If you looking for SharePoint 2013
then you can have sql express and don't need full sql version
in that case8GB with 4 cores can run basic sharepoint with sql express
if you need all services with full sql edition, you need 4 cores with 24 GB memory
https://technet.microsoft.com/en-us/library/mt493254(v=office.16).aspx

what I feel is, use single server with 16 Gb with 4 cores and increase memory based on usage

From pricing point of view:
For 20 users, I don't think you should go for onpremise solution
it costs 7000$ per SharePoint server (onetime investment)
CALs are 100$ per user
better you go with online model which save cost of hardware / software and it will be per user license
http://www.bemopro.com/Pricing/SharePoint.aspx
https://products.office.com/en-us/sharepoint/compare-sharepoint-plans

Thanks Mahesh, here is my issue. I have an accounting client that wants to be able to have their auditors work on client documents simultaneously. They are using the following products that need to be shared;
1. Caseware working papers
2. PPC Smart Practice Aids
3.The ability to work on Excel and Word files simultaneously.

The Caseware product has provisions to allow collaboration built in so that piece is not a big deal.
We are auditing municipalities and counties so I am reticent to have these documents residing on a public server, however there is a cost component to this. If we house on site we take the one time upfront hit but there is no recurring cost. Office 365 has been considered and apparently has some added benefits. What do you think?

you need to manage onpremise infra completely in terms of server hardware, backups, data, security, publishing, upgrades, patching, issue resolution in addition to management and its all only for 20 users
If you are abided with any specific data distribution law, then you need to invest with onpremise, then I am quiet

However Online SharePoint is also secure and works well and you can use built-in O365 technologies to add more security layer to it such as MFA, location based restrictions etc and you will save on all other factors mentioned above apart from management, so I would prefer online which is cost effective and handy

Thanks Mahesh, your comments were very informative