I'm not sure I understand this correctly - I know that it's impossible to run out of milk as it's constantly being produced, so how/why is it that introducing formula or feeding to too strict a routine can have a negative effect on the supply of milk? Surely if it's neverending, it's... never ending! I suppose it has to end sometime (e.g. when the child weans) but how can you end up with "supply problems" when there is still some stimulation there?
Because your breasts are constantly adjusting the supply to meet the demand, so that if your baby is going through a growth spurt, they produce more milk for the period that the baby is "demanding" more milk, and if you replace one breast milk feed with formula, then your breasts think "ah ha, less demand, better produce less milk".
Milk contains something called 'feedback inhibitor of lactation' (FIL). When milk is not removed fom the breast (for example because baby is getting formula instead, or feeds are restricted) FIL slows the production down.