TLDR: No general amount can be established based on the available sources. The €7,500 threshold concerns an income tax exemption, not a general exemption from social security contributions.
Answer
You cannot retain a single amount of remuneration not subject to social security contributions. The rules for calculating income tax and those for social security contributions are distinct, particularly for self-employed workers. [S003]
Scope of the €7,500 threshold
The annual threshold of €7,500 provided for by Article 81 quater concerns certain remuneration exempt from income tax. The source does not present it as an amount exempt from social security contributions. [S015]
Conclusion
You should therefore not equate the €7,500 threshold with remuneration not subject to social security contributions. The applicable amount would depend on a precisely identified scheme or situation, which is not determined by the available sources.