NUC
In the literature: open
Total number of particle-bound nuclei
In plain words
About 3,300 different nuclei have been observed, but theory suggests roughly twice as many could exist. The precise number depends on uncertain theory far from known nuclei.
Precise statement
Determine the number of nuclides with $Z \le 120$ bound against one- and two-nucleon emission, with uncertainty propagated from energy density functionals and ab initio calculations. Erler et al. (2012) estimated $6900 \pm 500$ using six Skyrme functionals. Answer: the count with model uncertainty below 300, and the regions of the chart dominating the uncertainty.
What would settle it
Global mass models with Bayesian uncertainty quantification calibrated on drip-line data for Z up to 20.