Welcome to The Gauge!
18" radius turns are considered the usual minimum in HO, although sectional track is available in 15" too. 18" and 22" radius sectional track allows for a double mainline loop to be made on a 4x6 or 4x8 tabletop.
Some newer models of big diesels and big steam engines require 22" or 24" curves to operate. With flex track, you have to be careful how you lay it, but any radius is possible from about 10-12" and up!
You've come to the right place - keep the questions coming!
Andrew