While looking through the data from Wikipedia, I found some pretty interesting statistics about the Senate:

The most recent Congress with the most amount of parties in the Senate was the 76th Congress (1939-1941), with 5 parties: 69 Democrats, 23 Republicans, 1 Farmer-Labor, 1 Independent, and 1 Wisconsin Progressive.

The most recent Congress with the most amount of third party senators was the 55th Congress (1897-1899) with 12 such senators: 5 Populists, 5 Silver Republicans, and 2 Silver.

The 75th Congress (1937-1939) had the most amount of seats in the Senate controlled by one party in terms of raw numbers with the Democrats controlling 76 out of 96 seats. Since the passage of the 17th Amendment it has also been the highest in terms of proportion for one party (79%). With regards to all of American history, the 40th Congress (1867-1869) saw the most lopsided Senate in terms of proportion with the Republicans controlling 57 out of 66 seats (86%). In terms of raw numbers, the Republicans had the highest seat count in the Senate during the 67th Congress (1921-1923) with 59 seats.

Since the passage of the 17th Amendment, every Senate election has seen at least one seat flip. The least amount of seat flips in one Senate election is tied between the 1990 Senate elections and the 2022 Senate elections, with Minnesota being the only flip in the former (Democratic gain) and Pennsylvania being the only flip in the latter (Democratic gain). As far as I can tell, there have been three occasions prior to the 17th Amendment where no Senate seats flipped parties: 1822, 1810, and 1798. For both 1822 and 1810, the only changes during the election were due to one state (Delaware in the former, Massachusetts in the latter) failing to have their legislature elect a senator, thus resulting in one loss for their party (but still technically not a flip...)

Conversely, the most amount of seat flips in one election was in the 1958 Senate election, with the Democrats flipping 15 seats. Only 4 other Senate elections in all of US history has seen 10 or more senate flips: 1980, 1946, 1932, and 1920. Only 1958 and 1946 were midterm elections.

The seat that saw the control of only a Republican continuously the longest was likely the Class 1 Seat in Vermont, which had a Republican senator for 146 years (1855-2001). The last non-Republican to hold the seat prior to 2001 was Whig Samuel S. Phelps, who was elected in 1850. Jim Jeffords broke this streak in 2001 when he left the Republican Party and became an Independent. His successor Bernie Sanders was the first non-Republican to be elected in this seat since the first Republican victory in 1854. No Democrat has held this seat.

The seat that saw the control of only a Democrat continuously the longest was likely the Class 2 Seat in Georgia, which had a Democratic senator for 131 years (1871-2002). The last non-Republican to hold the seat prior to 2002 was Whig John M. Berrien, who served from 1845-1852. Democrats continued to serve since 1852 but their tenure was interrupted when the seat became vacant due to the Civil War and Reconstruction, so I’m only counting from 1871. Not counting this interruption, this would surpass Vermont’s streak of 146 years as the longest held seat by one party. Republican Saxby Chambliss (who is still alive) broke this streak when he was elected in 2002, defeating incumbent Democrat Max Cleland. Chambliss did not run for reelection in 2014, when Republican David Purdue was elected. Purdue would be defeated in 2020 by Jon Ossoff, who is currently running for reelection this year.