Hello Kassandra,

I can't tell you an actual number, but I can assure you that all cultures built sites intended to honor/worship/observe the heavens. You probably know of Stonehenge, but there's also Avebury close by, which is a village inside a "henge". In Ireland, there are many sites, one of the most famous is Newgrange, megalithic tomb into which a shaft of light will fall at sunrise on the winter solstice. There's Namoratunga II, near Lake Turkana in Kenya, a site probably used to develop a calendar, there's the Caracol and El Castillo of Chichen Itza in the Yucatan (Maya/Toltec sites), and others, which I will simply list.

Peru: Machu Picchu, The Torreon, probably observed the Pleiades prior to winter (June 21 in the southern hemisphere) solstice sunrise.

In Cusco, the Qorikancha, a building dedicated primarily to the Sun, but also the stars and the Moon.

In Cusco, Saqsayhuaman, various structures for celebrating Inti Raymi, the winter solstice (June in southern hemisphere).

Northern France: La Roche aux Fees, the opening is aligned with the winter solstice (Dec. 21) sunrise.

Most ancient observatories were primarily concerned with observing the Sun and the Moon, but some did look for bright stars and planets.