Open orders refers to your maker limit orders that are waiting in the order book to be matched and filled. Scheduled orders are orders placed with a start time five seconds or more in the future and will count towards your maximum number of open orders.
The maximum number of orders you can have open at any given time (across each trading pair) is determined by your Verification level.
You have 78 buy limit orders BTC/EUR and 60 sell limit orders on ETH/EUR.
You want to place a few more buy orders on BTC/EUR, but you are only verified to Intermediate level. The maximum limit for open orders for Intermediate accounts is 80.
This means that you can place 2 more orders on BTC/EUR and 20 more open orders on ETH/EUR before you reach the maximum number of open orders on those pairs. On other pairs, you can place 80 orders each.