When starting out, it is best to stick to about a month's worth of inventory if possible and never more than 90-120 days worth of inventory. This cuts down on storage fees as well as reduces the risk by allowing you to sell out in a shorter time frame, leaving less chance for the product to potentially drop in price.
You can calculate a month's worth of inventory by using a sales estimator (AND Manual Estimation method) to estimate the sales and then dividing estimated sales by the number of sellers in the buy box + yourself. For example, if you estimate 300 sales a month according to jungle scout, and there are 2 sellers in the buy box, if you hop on as a 3rd seller, you can expect to sell about 100 units a month. So I would want to start with around 100 units.
This also can depend on the listing itself. If the product seems very consistent and low risk, you can consider more inventory. If the product seems like there are a lot of risk factors such as Amazon on the buy box in the past or lower price points in the past, then I would consider less inventory.