[SalesForce] Can we add a text value on number formula field

When using the INCLUDES funtion, you have to use literal text. See help docs tip below:

Tips: The text_literal expression must be of type text and enclosed in quotes. It cannot be a merge field or the result of a function. Salesforce returns an error if any of the following occurs: You do not provide a text_literal expression. You provide an empty text_literal expression, such as "" or " ". Use ISBLANK to determine if a multi-select picklist field is empty. Use the PRIORVALUE function inside the INCLUDES function to check if the previous value of a multi-select picklist field included a specific value. For example: INCLUDES( PRIORVALUE(multiselect_picklist_field), text_literal )

LastModifiedDate is not the correct field to check here. Consider a scenario where the picklist value is set to OPENED 5 days back. A sales user goes ahead and edits this opportunity 2 Days back for some other fields, Now your formula will yield result as 2 which is incorrect. Instead -

Create a Custom Field "Stage Opened Date" of type Date.
Create a Workflow rule that fires if ISPICKVAL( StageName , "OEF Clarification required stage")
In W/f action add a field update on this new field and populate as TODAY()
Create a Formula - Date Since Opened return type Number and Formula : TODAY() - Stage Opened Date

PS: you get error in above formula as TODAY() is a Date field and LastModifiedDate is a datetime field, Use DATEVALUE function to convert a date/time field into a date field.