Why if i pay for a basic store they only gave me 400 items to list?

[jame234]

You are a new seller and new sellers have selling restrictions.  Subscribing to a store does not increase those limits.  The limits usually last 3 months.  Review your store subscription and go to my eBay Summary page to see how many listings you have.

[woodland_gnome]

The limit on the  number of items you can list in a month  and your allowance of free basic insertion fees per month are two entirely separate limits, each applying on its own terms.   If you max out the former you cannot list anymore until the month is over.  If you max out the latter you can still list items (as long as you haven't maxxed out the former) but will be charged the basic insertion fee for each one over the limit (note that for a Basic Store there are different allowances for auction vs. fixed price listings and it varies by category:   https://www.ebay.com/help/selling/selling-fees/store-fees?id=4809#section2  )