Astanga Hrudaya
रसानां यौगिकत्वेन यथास्थूलं विभज्यते। एकैकहीनास्तान् पञ्चदश यान्ति रसा द्विके॥४०॥
on the basis of their usage (in daily routine of selection of drugs, planning of therapies etc.) Eliminating 1 rasa at each combination , the number of combination of 2 rasas each will be 15 In the combination of 3 rasas each , it will be 10 with svadu, 6 with amla, 3 with lavana, 1 with tikta total 20
english translation
rasAnAM yaugikatvena yathAsthUlaM vibhajyate। ekaikahInAstAn paJcadaza yAnti rasA dvike॥40॥
hk transliteration by Sanscript