How much is too much???

Hi! We have a 9 week old puppy and we have started training her. My question is how many commands should we be working on at one time? I mean, do we only focus on her learning the "sit" command and when she masters it we should move on or should we focus on learning 2 or 3 commands at the same time (not doing he 2-3 commands in one training session, but maybe the sit command in the morning training and the down command in the afternoon)? And teaching her these commands while learning to leash walk, etc? I don't want to overwhelm her....or me.

Thank you in advance for your help!