How Many Topical Themes Per Clause?

In 1925	Halliday	was	born
Theme: marked	Rheme
Location	Goal	Process: material
circumstantial Adjunct	Subject	Finite	Predicator

Halliday & Matthiessen (2014: 91, 105, 107):

As a general guide to start off with, we shall say that the Theme of a clause is [realised by] the first group or phrase that has some function in the experiential structure of the clause, i.e. that functions as a participant, a circumstance or the process. …

The guiding principle of thematic structure is this: the Theme contains one, and only one, of these experiential elements. This means that the Theme of a clause ends with the first constituent that is either participant, circumstance or process. We refer to this constituent, in its textual function, as the topical Theme. 

There may however be other elements in the clause preceding the topical Theme. These are elements that are either textual or interpersonal in function, playing no part in the experiential meaning of the clause.

Beware of those who do not understand the distinction between the highlighting of a functional element (Theme) and functional element that is highlighted (e.g. Subject, Senser, etc.).