The theoretical answer is that there is no absolute upper limit, and the non-theoretical impractical answer is 10. The practical answer is 5-6, depending on why you really care. You describe your setup articulatorily, but formants are acoustic objects. I presume that you are using articulatory settings to somehow synthesize waveforms. If so, you could be more or less doing this as source-filter synthesis, or as lpc synthesis. Since some crucial details are lacking, I can only talk in general terms, from the perspective of source-filter synthesis. As it happens, I've been exploring some problems in that area, aiming to extract reliable formants for expertly-produced IPA vowels.

The number of formants that would be present is relative to a frequency range (e.g. "up to 5Khz" versus "up to 8Khz"). In the case of humans, 5K is approximately "what you would care about" for adult males, and for people with shorter vocal tracts, maybe 8K (children – dunno what would be correct for newborns). A conventional number for adult females is 5.5K. Given a target frequency range, you have to pick the right number of formants, i.e. 5, 5.5 or 6 (maybe 4.5 or 6.5, that remains to be seen). You basically pair the right number of formants with the right frequency range. If you are wrong, you may merge formants (F1 and F2 will not be distinguished for [u,o]) – not enough formants – or you may grow spurious formants – too many formants. You don't want the maximum possible number of formants, you want the right number of formants. I hope to know what number of formants and frequency range gives a "correct" answer for an IPA chart of 3 experts within a few days. I do know however that even the experts vary massively for what is ostensively the same vowel.

(The "5.5 formants" oddity is due to the way LPC analysis works: you need twice the number of LPC coefficients as desired formants, since the coefficients are pole-zero pairs. So saying 5.5 formants translates into 11 coefficients).

The theoretical answer is that there is no absolute upper limit, and the non-theoretical impractical answer is 10. The practical answer is 5-6, depending on why you really care. You describe your setup articulatorily, but formants are acoustic objects. I presume that you are using articulatory settings to somehow synthesize waveforms. If so, you could be more or less doing this as source-filter synthesis, or as lpc synthesis. Since some crucial details are lacking, I can only talk in general terms, from the perspective of source-filter synthesis. As it happens, I've been exploring some problems in that area, aiming to extract reliable formants for expertly-produced IPA vowels.

The number of formants that would be present is relative to a frequency range (e.g. "up to 5Khz" versus "up to 8Khz"). In the case of humans, 5K is approximately "what you would care about" for adult males, and for people with shorter vocal tracts, maybe 8K (children – dunno what would be correct for newborns). A conventional number for adult females is 5.5K. Given a target frequency range, you have to pick the right number of formants, i.e. 5, 5.5 or 6 (maybe 4.5 or 6.5, that remains to be seen). You basically pair the right number of formants with the right frequency range. If you are wrong, you may merge formants (F1 and F2 will not be distinguished for [u,o]) – not enough formants – or you may grow spurious formants – too many formants. You don't want the maximum possible number of formants, you want the right number of formants. I hope to know what number of formants and frequency range gives a "correct" answer for an IPA chart of 3 experts within a few days. I do know however that even the experts vary massively for what is ostensively the same vowel.

(The "5.5 formants" oddity is due to the way LPC analysis works: you need twice the number of LPC coefficients as desired formants, since the coefficients are pole-zero pairs. So saying 5.5 formants translates into 11 coefficients).

The upper limit on formants is based on facts of speeech, that is, we don't care from an informational perspective about the 6th formant value. The acoustics of singing is different enough that I don't think you can translate speech-centric settings to singing-specific settings. Filtering out (discounting) high frequencies could be counterproductive if the goal is synthesized singing voices that are aesthetically successful.