I have created Custom fields for Inventory Items (Brand) and Custom Field for customers (Salesperson) (Branch)
How can i get a report with these Custom fields included, for example i need to know which sales person sales of each brand. As we cannot include more than one custom field per report
Not sure what you are trying to say here. See Use custom fields | Manager specifically the list under “…Custom fields can be placed on records and transactions for most functional tabs enabled in the navigation pane…”
Note that these records and transactions for example Sales invoice can each contain more than 1 custom field, so not sure what you refer to, to see these you need to edit columns and enable their view in the tab-listing.
There is no mention of reports, none of the build-in reports allows custom fields and I do not think they work either with Custom reports as referred to by @Syed_Salman_Ali but you could try and show the results here.
Custom fields can only be used do not show in the build in reports