I mostly know common-practice theory, and only the broad strokes of jazz and rock harmony. Be advised that the interpretation of a C major chord as potentially an Em6 is not generally accepted or discussed in classical terms. However, a move from a root position chord to a first inversion chord with a root a third away by moving the fifth up a step is quite common. For example, a C major chord (C–E–G) often becomes an "a minor chord" in first inversion (C–E–A), especially as the setup to a 7–6 sequence. The move is often called 5–6 (named from the typical figured bass symbols), and many common-practice theorists, especially Schenkerians, do NOT consider the second chord to truly be a different harmony than the first. If my example happened in the key of C major, they generally wouldn't label them as two chords, I–vi6, but just as a prolongation of I, I5–6. Although the terminology is different, that actually isn't all that different from jazz and pop ideas about 6 chords, wherein E–G–C might indeed still be interpreted, in a sense, as an Em harmony.
At any rate, more specifically responding to your question, every triad in first inversion could theoretically have a second "interpretation" as a 6 chord in root position. The second inversion is even shiftier: in most classical situations the second inversion of any triad—due to the dissonant interval of a fourth above the bass—functions very differently from its root-position counterpart. For example, in the key of C major, a C major triad in second inversion could function as a passing chord (IV–I6/4–IV6), a pedal or neighboring chord (V–I6/4–V) or as a dissonant embellishment of V called a Cadential 6/4 (V6/4–5/3). Every diatonic chord could function as a passing or pedal 6/4, but only the "I" chord can be Cadential.
So, in the sense that changing inversion changes a chord's interpretation, every diatonic harmony has at least 4 interpretations. It can be itself, a 6th chord in the jazz sense, a passing 6/4 or a pedal 6/4. The I chord has a fifth standard interpretation as a Cadential 6/4. I suppose for completeness sake we could also say that every diatonic harmony has one more possible interpretation as a mere link in a sequential chain.
The other half of your question is about functions within a key. Aside from the subtler shifts wrought by inversion discussed above, most diatonic and chromatic chords within a single key have only one standard function. V chords tend to always behave like dominant-function harmonies, ii6 tends to always be pre-dominant, etc. The most common exception among diatonic harmonies is vi, which can often have weak pre-dominant function—tending to move to a stronger pre-dom like IV or ii—or tonic function as the goal of a deceptive cadence (V–vi). iii is a protean harmony (and not very common in major keys) that occasionally has weak dominant or tonic function depending on context. As part of a circle of fifths sequence it can even be a weak pre-dominant.
Otherwise, chords tend to only have one standard functional interpretation (again, ignoring the complexities discussed in my second paragraph) so long as you stay within a single key. When modulating however, it is quite common to use two different interpretations of some pivot chord to effect the transition. For example, if I'm in the key of C major, a C major triad is my I chord. If I suddenly start treating it like a IV chord in the key of G major, I can make a sneaky modulation into the new key. Re-interpretation of the triad is precisely how I'm able to make the smooth shift.