Our query is that whether one M S OFFICE volume licence is sufficient for above server so as to use it with the help of RDC and User cals on these client PCs.
Each end-user-device (workstation) must be licensed for Office, if you are using Office via RDS, if the Office edition is not Office365ProPlus.
https://www.microsoft.com/en-us/licensing/learn-more/brief-remote-desktop-services.aspx
NB: If the Office edition you are using is Office365ProPlus, that is a per-user Subscription license.
Either way (Subscription or Perpetual), if you have 18 users, or 18 workstations, you will need 18 Office licenses.
The RDS server itself does not require an Office license at all, for a simple scenario.
http://download.microsoft.com/download/6/B/A/6BA3215A-C8B5-4AD1-AA8E-6C93606A4CFB/Windows_Server_2012_R2_Remote_Desktop_Services_Licensing_Datasheet.pdf
[Page 4 - FAQ]
Q: How do I license Microsoft Office in an RDS environment?
A: If you are accessing any proprietary application through your VDI infrastructure, it may require a license in addition to the RDS license.
Microsoft licenses its on-premises desktop applications on a per-device basis.
Per-device licensing means a customer must obtain a license for each device on or from which the product is used or accessed.
For example, when a desktop application is accessed remotely across an organization using Windows Server Remote Desktop Services, a separate desktop application license is generally required for each device from which the application is accessed.
https://www.microsoft.com/en-us/Licensing/product-licensing/windows-server-2012-r2.aspx#tab