What simulator do you use ?
Spectre. I think I know basics about harmonic balance, may not be expert. I usually keep number of harmonics and over sampling factor to their default values auto and 1 respectively.
There is information here
https://community.cadence.com/cadence_blogs_8/b/rf/posts/setting-up-harmonic-balance-part-1. But I can't really relate it to loadpull and optimum impedances.
Number of Harmonics (MxHam): This is the number of harmonics of currents at each node that the simulator will use find a "balanced" solution. The trick is to use enough harmonics to allow the simulator accurately create the waveforms. But, the more harmonics you have, the larger the number of unknowns and the more memory and simulation time required.
The number of harmonics is the primary parameter controlling accuracy for Harmonic Balance
Setting Mxham correctly is important. Too little and you can get incorrect results and convergence problems. Too many increses simulation time and memory used.
Start with 5-8 harms on large tones and 3 harms for linear tones
For strongly nonlinear tones (I.E. square wave LO, VCO with divider), use more harms. Can improve convergence. 10-20 harms is recommended.
For circuits with frequency divider, we suggest harms= 5 x divide_ratio for that tone.
Periodically increase # of harms for key tones to see if answer changes. If not, it means the # of harmonics is sufficient.
Oversample (Ovsap): Oversample helps the simulator more accurately represent a nonlinear waveform by adding more samples or you can think of more time points to the waveform.
For most cases, use Oversample = 1 (default)
For strongly nonlinear tones, set Oversample = 2 and you can reduce the # of harms required
Does a better job recovering the time domain waveform for IFFT.