Is it better to donate for the construction of one mosque or to contribute to the renovation of several mosques when giving an ongoing charity (Sadaqah Jariyah) on behalf of the deceased?

It is best to consider the option that is more in need, as both are ongoing charities (ṣadaqah jāriyah). One can seek Allah's guidance (istikharah) regarding which of the two options the soul inclines towards. Also, charity can be spent on any of the avenues of righteousness.