Minimum radius. Really broad rule of thumb, minimum radius of 3 times then length of your cars.
Remember minimum radius is a MINIMUM, that is the bottom below which you aren't going to go. Standard radius is the one you shoot for for most applications. The minimum radius recommended by the manufacturers is a harder bottom. That is the radius below which the model starts to not work well. The risk with going with the minimum radius is if you aren't really consistent with your track laying, its possible to end up BELOW the minimum and that could result in the track not operating well.
Your track will be more reliable if you set a minimum and then try to stay above that, for example saying 18 is the minimum and trying to stay at 19-20 in radius will end up being way more reliable.
As far as switches go, a number 6 will handle about 95% of all the model railroad equipment. A number 8 looks really cool, but takes up a lot of room. Figure a switch in HO is as long as twice the frog number. A number 6 switch is nominally a foot long. A #6 crossover is about 2 ft long.
Another thing to watch is track centers. The tighter the radius, the more "swing out" and "overhang" cars will have. With 2" track spacing on 18" + 20" radius, running long cars (passenger or auto racks) may result in the cars hitting each other on the curves. For really tight curves allow an extra 1/4 to 1/2 inch of track spacing (ex. 18" and 20 1/2" radius) to make sure your cars won't sideswipe each other. Once you get over 24-27" radius its not as critical, or if you are only running 40 ft or shorter cars its not as critical. For example I model 1903 and the longest cars I have are 50-60 ft passenger cars and most freight cars are in the 34-36 ft range. For me having 2" spacing on 20" radius curves, not a problem. However if I tried to run 89 ft piggyback cars, auto racks and SD90's by each other, there could be issues.
(OOPS, just noticed you are in N scale, completely changes the numbers, but the concepts are the same.)