For publication bias study in a meta-analysis by funnel plot test, there should be at least 10 studies, fewer studies might not give sufficient power to the test and may not detect real asymmetry. Is this statement correct?
You can check for the answer in https://handbook-5-1.cochrane.org/
From the handbook, it states that "Recommendations on testing for funnel plot asymmetry
For all types of outcome:
- As a rule of thumb, tests for funnel plot asymmetry should be used only when there are at least 10 studies included in the meta-analysis, because when there are fewer studies the power of the tests is too low to distinguish chance from real asymmetry.".
So to answer your question, you must assess for the publication bias even if there are only 2 articles but for the statistical test for funnel plot asymmetry, n should be more than 10.
There are many ways to identify bias in the systematic review. When I do the systematic review, I always find articles that say "there is no association between A and B (p < 0.05)." and that's it. This indicates selective reporting bias. Please see https://methods.cochrane.org/bias/reporting-biases
Hope this help!
https://training.cochrane.org/handbook/current
The current version is Version 6.2, 2021 above. For your question, go over the statistics that would give you 95% confidence (p < 0.05), which will give you a rough estimate for your purpose.
The more you add, the more validity you will obtain, and reduce the biased assessment.