In the manual of xterm, you can find the description of the relevant option:
-sl number This option specifies the number of lines to save that have been scrolled off the top of the screen. This corresponds to the saveLines resource. The default is “64”.
So you can call
xterm -sl xxx
or create an alias
xterm='xterm -sl xxx'
or set saveLines in your (or the global) Xresources file.