Your answer is 'the amount of thrust required to move the Aircraft'
Stilton provides the best answer, however there are guidelines. Typically around 25-30% N1 will get you moving on a level surface and idle thrust keeps it going until after a brake and turn. It may take a few seconds at lower settings to get rolling. Somewhere I have seen the 35% limit number for taxi breakaway, but cannot recall the source - perhaps it comes from CFM maintenance data?
The only guidance I know from Boeing is a warning against >50% N1 during single-engine taxi - two primary reasons - FOD ingestion and jet blast concerns.