There is a seasonal variation for the mean number of seeds per plant. Further, all seeds are not able to germinate. There are some seed which are defective in viability.

This is a more or less practical question. Since you are not specific on which species, you are asking about the genus Gamochaeta. There are about 56 species under this genus, the total of seeds from the 56 species divided by 56 will give the mean number of seeds per plant for that genus. But if you are referring to a particular species, the total of seeds of that species collected from different locations in your research area divided by the number of plant stands in the research will give the mean number of seeds per plant for the species. For further info check Flann, C (ed) 2009+ Global Compositae Checklist

I share the views of Professor  Yakubu Pwajok Mwanja