As in sexual affairs? If so,
The only affair that can be confirmed is one that took place between Marilyn and her french Let’s Make Love (1960) co-star Yves Montand.
In general, there was countless rumours of affairs. The most famous alleged affairs occurred with President John F. Kennedy and his brother Robert “Bobby” Kennedy. Although, these were just baseless rumours. There was never any proof of any affairs: Remember, that at THAT TIME being an“easy" woman, aka sexually active, was frowned upon. And would not have been tolerated by the movie studios.