An inventory equal to 90 days sales is as much as even the strongest businesses carry, and then only as a way to anticipate higher prices or ensure against shortages.
what does the sentence mean?
what is a "businesses carry?"
Inventory is product that a company keeps in stock. The amount of inventory that a business "carries" is the amount that they have on their premises available for immediate sale. If they carry more than they can sell in a set amount of time (90 days in this case), they could lose money. If they anticipate a sudden surge in sales of an item, they may stock more product (for example, in early Spring an appliance store will try to keep more air conditioners in inventory than usual, because the demand for them will be high during the Summer months).