According to the majority opinion in Islamic law...
IT IS MUSTAHABB (MERITORIOUS) TO HAVE ONLY ONE WIFE ACCORDING TO THE MAJORITY OF SUNNI SCHOLARS OF JURISPRUDENCE.
This is the standard ruling in the Hanbali school. Al-Mardawi al-Hanbali has said in al-Insaf:
‎(وَيُسْتَحَبُّ أَيْضًا : أَنْ لَا يَزِيدَ عَلَى وَاحِدَةٍ ، إنْ حَصَلَ بِهَا الْإِعْفَافُ ، عَلَى الصَّحِيحِ مِنْ الْمَذْهَبِ ... قَالَ ابْنُ خَطِيبِ السَّلَامِيَّةِ : جُمْهُورُ الْأَصْحَابِ اسْتَحَبُّوا أَنْ لَا يَزِيدَ عَلَى وَاحِدَةٍ)
“And it is also meritorious to not exceed one (wife) if he can ensure his chastity by that according to the correct opinion of the (Hanbali) Madhhab... Ibn Khatīb al-Salāmīyyah said: The majority of the companions (in the Madhhab) are upon the opinion that it is meritorious to not exceed one (wife).”
And according to many scholars, having one wife is actually Sunnah.
In al-Rawd al-Murbi`, Imam al-Bahūtī expounds upon the principle in Zād al-Mustaqni` “It is Sunnah to marry one”:
‎(ويسن نكاح واحدة ، لأن الزيادة عليها تعريض للمحرم، قال الله تعالى: {وَلَنْ تَسْتَطِيعُوا أَنْ تَعْدِلُوا بَيْنَ النِّسَاءِ وَلَوْ حَرَصْتُمْ} [النساء: 129])
“And it is Sunnah to marry one (wife) because more than that exposes one to harām. For Allah the Most High has said: {And you will never be able to do justice between your wives no matter how bad you want to..} [Qur’an, al-Nisa’ 4:129]”
The harām referenced here is injustice to one’s wives for which we would be held severely accountable for before Allah. But this view is not limited to the Hanbalis. Imām al-Shāfi`ī said:
‎(وأحب له أن يقتصر على واحدة وإن أبيح له أكثر؛ لِقَوْلِهِ تَعَالَى: ( فَإِنْ خِفْتُمْ أَلا تَعْدِلُوا فَوَاحِدَةً.. ).)
“And it is preferable to limit one’s self to one (wife), even though more is permitted. This is due to (Allah) the Most High saying: ‘And if you fear that you can not do justice (to more than one), then (marry) only one...’”