Linux commands are not limited by number.there are linux command references in Linux distros itself.
*cfa-www.harvard.edu/~jbattat/computer/linuxReferenceCard.pdf
dont expect a black screen like in cmd in windows.u can learn commands(if necessary) after installing Linux only!also Linux by default dont expect u to learn commands.it is,when some unsupported devices,particularly nvidia,ati or onboard VIA unichrome gfx cards,wifi cards etc and for some complex networking etc that u need commands.
For reference or to download pdf.
goto *tldp.org if u want to know about different shells like bash,sh,tcsh etc.these are all not necessary.just start without any expectation of Linux to contain setup.exes,program files dir etc as in windows.instead Linux file system is different which u can see and learn after installing ubuntu for ur distro.if amd64 processor,download gutsy 7.10amd64 edition.
ps:some common cmds are ls(show list),mv(cut,move),cp(copy),rm(remove/delete),man(manual for command).