What do the ‘onion’ domes of Russian churches signify?
One dome symbolizes the One God. Temples with one dome were often built even before the Mongol invasion. For example, the amazing beauty of the Church of the Intercession on the Nerl in Vladimir Region was erected in the 12th century in honor of the victory of Prince Andrei Bogolyubsky over the Bulgars and in memory of his son Izyaslav Andreyevich.
Churches with two domes are rare and symbolize the nature of the Savior: human and divine. For example, you can find the two-domed Church of the Assumption of the Virgin is, in the Annunciation Monastery in Nizhny Novgorod. In Moscow, there is also such a church - the Church of Cosmas and Damian, built in 1564.
Finding one with three domes is easier: they denote the Holy Trinity. You can find many of them in the Rizopolozhensky Cathedral in Suzdal, for example. Five domes, meanwhile, as in the Assumption Cathedral of the Moscow Kremlin, remind of Christ and the four evangelists.
Seven domes represent the symbols of the sacraments of the Church and adorn the main cathedral of the Don Cossacks in Novocherkassk. And nine, according to the number of angelic ranks - St. Petersburg Savior-on-Blood.
Thirteen domes, denoting the unity of Christ and his apostles, can be found at the Church of the Transfiguration of the Lord in Zvyozdny gorodok.
And you can find churches with even more domes. For example, there is the 25-dome (!) Church of the Intercession of the Virgin Mary in Nevsky Forest Park. Each symbolizes the glorification of the Mother of God by the 25 prophets of the Old Testament.
According to the number of years of Christ's earthly life - 33 domes can be found at the Transfiguration church of Ust-Medveditsky Spaso-Preobrazhenskiy Monastery, located in Volgograd Region.