How many days are there in one Yuga (each of the four Yugas)?
I'm not sure why you're interested in the number of days as opposed to the number of years, but in any case it's simple arithmetic. Here is how the Vishnu Purana describes the lengths of the four Yugas:
They are thus distributed: the Krita age has four thousand divine years; the Tretá three thousand; the Dwápara two thousand; and the Kali age one thousand: so those acquainted with antiquity have declared.
Now we need to convert divine years into human years, and then human years into human days. Thankfully the Vishnu Purana gives us the relevant information immediately before the quote above:
Thirty Muhúrttas constitute a day and night of mortals: thirty such days make a month, divided into two half-months: six months form an Ayana (the period of the sun's progress north or south of the ecliptic): and two Ayanas compose a year. The southern Ayana is a night, and the northern a day of the gods. Twelve thousand divine years, each composed of (three hundred and sixty) such days, constitute the period of the four Yugas, or ages.
So a year of the gods is 360 days of the gods, and a day of the gods is 1 human year. So we need multiply the above figures by 360 to get the length of the Yugas in human years: the Satya Yuga is 1,440,000 years, the Treta Yuga is 1,080,000 years, the Dwapara Yuga is 720,000 years, and the Kali Yuga is 360,000 years. If we factor in the the Sandhya and Sandhyansa periods that come before and after each Yuga, we get that the Satya Yuga is 1,728,000 years, the Treta Yuga is 1,296,000 years, the Dwapara Yuga is 864,000 years, and the Kali Yuga is 432,000 years.
Now a human year is said to be two Ayanas, which works out to 360 days. So if we convert the above figures to days, we get that, the Satya Yuga is 622,080,000 days, the Treta Yuga is 466,560,000 days, the Dwapara Yuga is 311,040,000 days, and the Kali Yuga is 155,520,000 days. Whew!
The number of years varies in the four yugas as below:
The duration of the Satya millennium equals 4,800 years of the years of the demigods; the duration of the Dvapara millennium equals 2,400 years; and that of the Kali millennium is 1,200 years of the demigods. [SB - 3.11.19]
And calculating those god years in human years (1 god year = 360 human years) and similarly taking one year roughly to be 360 days would have the following values:
Satya Yuga - (1,728,000 solar years) - 622,080,000 days
Treta Yuga - (1,296,000 solar years) - 466,560,000 days
Dvapara Yuga - (864,000 solar years) - 311,040,000 days
Kali Yuga - (432,000 solar years) - 155,520,000 days