I am conducting a meta-analysis in the ecology discipline, such as on the effect of elevated CO2 on soil organic carbon. In my case, treatment and control groups have different sample sizes within the same study. For example, in 5 studies, combined control n is 43 and treatment n is 90. So, what will be the sample size for the overall meta-analysis? In ecology, the sample size of the mean effect size has only been reported if you study the multiple factors (e.g., elevated CO2, N fertilization, etc.) in the same paper. In other words, a forest plot is separately produced without including individual studies.
The point of meta-analysis is not to aggregate all the data together in one lump but to summarise in some way study-specific effects. In that context reporting the number of studies is best. If you have all the original data and do an appropriate individual participant data meta-analysis then report the number of participants but otherwise it is misleading for a conventional meta-analysis.