The question, "How much inventory should I have?" directly relates to the place element of the marketing mix.

Inventory

The total amount of goods or materials contained in a store or warehouse at any given time.

Place Element

One of the four Ps in the marketing mix, referring to the distribution and accessibility of a product or service to the target market.

Marketing Mix

The set of controllable marketing variables, traditionally identified as product, price, place, and promotion, that a company uses to pursue its marketing objectives.