Technically, ANY number of related records in a child table can be shown in a portal, but a portal isn't necessary when there's a 1 to 1 relationship. So you are correct.
You can use Go To Related Records script step to open a new list-view layout and show those related records.
That's all I know. The ways you listed and what I added, i think, are common ones.
Use the List ( ) function to assemble a list of the related data and display it in a single text field.
Use HTML to assemble a table and display the related data in a Web Viewer object.
What is the purpose / end use for the display of related records? That will determine your choice of method.
Is there any other technique to display more than one related record.
What do you mean by "display"? Display on screen? Display in a PDF report? Display on a dashboard?
In other words, who is the end user of this display and how will s/he consume (i.e. view) the data?
In general, I just meant to display in the screen.
I did some analysis activity and tried to find out different ways of displaying the related record.
Also, ExecuteSQL using current record as a parameter in WHERE clause. You can set a field to the result or even use a merged variable on a layout. But again, what is wrong with using a portal object?