Short answer
One. Muslims believe in one God, Allah, who has no partner, no son and no equal. This belief, called tawheed (the oneness of God), is the first article of faith and the meaning of the shahada: there is no god but Allah.
Belief that Allah is one in His essence, His attributes and His actions is the foundation of iman (faith), and associating any partner with Him (shirk) takes a person out of Islam. This is the agreed creed of all Sunni schools: Maturidi, Ash'ari and Athari.
The four schools agree on this.
Explanation
Islam is strictly monotheistic. Allah is one in His essence: He is not made of parts and has no partner. He is one in His attributes: nothing shares His knowledge, power or life in the way He has them. And He is one in His actions: He alone creates, gives life and causes death. So every act of worship, such as prayer, sacrifice and vows, belongs to Him alone.
"Allah" is not a separate god of the Muslims. It is the Arabic word for the one God, and Arabic-speaking Christians use it too. Muslims believe He is the same God who sent Nuh, Ibrahim, Musa and Isa, peace be upon them. What Islam rejects is the belief that God has a son or is three in one (Quran 4:171).
Surah al-Ikhlas sums up the whole belief in four lines: Allah is One, the Eternal Refuge, He neither begets nor is born, and nothing is equal to Him. The Prophet ﷺ told Mu'adh ibn Jabal that the right of Allah over His servants is that they worship Him and associate nothing with Him.
The Quran also gives a rational proof: if the heavens and the earth had several gods, they would have fallen into ruin (21:22). Two all-powerful wills would clash, and the order we see shows a single Lord. The classical Hanafi creeds, al-Fiqh al-Akbar and the creed of Imam al-Tahawi, both open with this point. For the full picture, read our guide to tawheed, shirk and kufr.
The evidence
Quran, Surah Al-Ikhlaas 112:1-4
بِسْمِ ٱللَّهِ ٱلرَّحْمَٰنِ ٱلرَّحِيمِ قُلْ هُوَ ٱللَّهُ أَحَدٌ ﴿١﴾ ٱللَّهُ ٱلصَّمَدُ ﴿٢﴾ لَمْ يَلِدْ وَلَمْ يُولَدْ ﴿٣﴾ وَلَمْ يَكُن لَّهُۥ كُفُوًا أَحَدٌۢ ﴿٤﴾
Say, "He is Allah, [who is] One, Allah, the Eternal Refuge. He neither begets nor is born, Nor is there to Him any equivalent."
Surah al-Ikhlas defines Allah as one, needing nothing, with no child, no parent and no equal.
Quran, Surah Al-Baqara 2:163
وَإِلَٰهُكُمْ إِلَٰهٌۭ وَٰحِدٌۭ ۖ لَّآ إِلَٰهَ إِلَّا هُوَ ٱلرَّحْمَٰنُ ٱلرَّحِيمُ ﴿١٦٣﴾
And your god is one God. There is no deity [worthy of worship] except Him, the Entirely Merciful, the Especially Merciful.
States directly that there is only one God worthy of worship.
Quran, Surah Al-Anbiyaa 21:22
لَوْ كَانَ فِيهِمَآ ءَالِهَةٌ إِلَّا ٱللَّهُ لَفَسَدَتَا ۚ فَسُبْحَٰنَ ٱللَّهِ رَبِّ ٱلْعَرْشِ عَمَّا يَصِفُونَ ﴿٢٢﴾
Had there been within the heavens and earth gods besides Allah, they both would have been ruined. So exalted is Allah, Lord of the Throne, above what they describe.
Gives the rational proof that more than one god would bring ruin to the universe.
Sahih al-Bukhari 5967
Sahih... حَقُّ اللَّهِ عَلَى عِبَادِهِ أَنْ يَعْبُدُوهُ وَلاَ يُشْرِكُوا بِهِ شَيْئًا ...
"The right of Allah over His servants is that they worship Him and do not associate anything with Him."
Shows that worshipping one God alone is the core duty of every person.
Where the ruling is recorded
- al-Fiqh al-Akbar (Imam Abu Hanifa), Opening section on tawhid. Opens by affirming that Allah is one, not by number but in that He has no partner.
- al-Aqida al-Tahawiyya, Opening section on the oneness of Allah. Begins: Allah is one, without partner, nothing is like Him and nothing is beyond His power.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.