This is not an easy question as there are many items to consider. Management structure, wages, job descriptions, benefits offered and more. Turn over is a costly part of business, instead of asking the question what is acceptable, I would ask what is not acceptable. Make changes, get insight, conduct exit interviews and find out why you seem to have turn over. Make adjustments accordingly.

Furniture will take more space in werehouse. Aldo these furniture is very slow moving product. It will stay in your werehouse for a long time. So i wont advise to keep many furniture in your stock.

On the other hand office stationery is a fast moving product. They dont rake more space. And so you can keep a large amount of them in stock.

There is a formula in Operations Research which exactly tells you the desired inventory size based on the monthly/annually shipments runrate and the acceptable backlog. Look at the inventory theory here https://www.ime.unicamp.br/~andreani/MS515/capitulo12.pdf