I guess it varies from person-to-person;
What can you afford to stock, do you have someone who can restock without hassle, etc.
For instance, in contemplating my stock I took into account:
1) I'm new here, it will start slow.
2) I have methods to regenerate stock easily.
3) I do not allow orders over 150M.
However, this isn't applicable to everyone. If there is one thing I have learned in my quest for world domination; everyone's needs are different and must be catered specifically for said person.