When new order is created, then new number (not id) is automatically generated and assigned to it.
Also there is a configuration variable in In-Commerce, that stores last generated order number, that doesn't work actually (tested on 5.1.2-B2).
Since special configuration variable isn't used, then last order number from database plus 1 is assigned to each new order. That doesn't pose any problems on Front-end, since every Front-end customer will use the same Orders table.
However it poses a serious problem to Admin Console. When admin opens new order creation page, then a number is generated and assigned to it. But the order record is created in a temporary table and that's why order number generator returns the SAME order number in case if 2 admins will open new order creation page in Admin Console.
Who saves his order first wins. And other admin will get "duplicate order number" message and is stuck with inability to save an order.