Hi, no it's not possible to buy only Nintex Forms licenses for Office 365.

Hi Vaibhav,

I see you've already created a support ticket for this, and one of my colleagues will get back to you.

For the benefit of anyone else:

How are allowances calculated for Nintex Forms?
For each workflow purchased, a customer will have an allowance of five (5) standalone Nintex Forms.
A standalone form is considered a form that is not backed by a workflow process (Start Form) or part
of a Nintex Workflow Action.
For example, if a customer purchased 5 workflows, they would be able to create 25 list forms using
Nintex Forms.
Will a workflow that has a start form and some task forms impact the forms allowance?
No, only Nintex Forms for SharePoint Lists or Document Libraries that do not initiate a workflow
process are counted.
Will Nintex Forms for Office 365 that start a process be included in a license count?
No. Nintex Forms for Office 365 does not have true ‘start forms’.
What happens if multiple Nintex Forms are associated to multiple content types in SharePoint
/ Office 365? How are these counted?
If a customer has multiple start forms associated to different content types in SharePoint / Office
365, these are considered individual Nintex Forms and will be counted towards the Allowable Forms
Limit.
Are Start Forms included in the license counts?
No, a start form that feeds a workflow process is not subject to licensing counts.
Are Task Forms included in the license counts?
No, Task forms created as part of a task action are not subject to license counts.
Are List Forms published to Nintex Live subject to additional license counts?
No, enabling a List form for Nintex Live will not accrue any additional licensing.
Are there any license considerations if a customer creates more than one layout for a Nintex
Form?
No, customers can have as many layouts as they wish for a specific Nintex Form without impacting
the licensing.

Are there any additional licensing considerations if a customer chooses to publish a Nintex
Form into Nintex Mobile?
No, publishing a form to Nintex Mobile is considered the same as publishing a layout (above).
What about Nintex Forms with a small number of controls? Is there a limited control form
concept?
No.

Source

However do contact us at sales@nintex.com for the latest pricing information.