I guess the previous post has covered the basics, you are probably runway limited, but do look at the range graph too, that will help you decide how many seats that aircraft can take on that route.
Then have a look at the potential First and Business Class traffic and configure your planes seating to meet that demand first, then add enough Tourist Class to meet the total demand. The standard seating is OK, but for long range traffic the Lux Seating will be appreciated by slightly better prices, though it doesn't suit most routes unless, as you are on this route, capacity limited by range.
Those premium classes pay well once the demand is there, you may only get 2 Business seats in the space of 3 Tourist but the pay rate is double, and double again for First.
By reducing the seat numbers there is much better passenger satisfaction, and a bit of airline public rating, which increases seat prices significantly. Having a few spares seats is best, it makes possible competitors choose less contested routes for one, but you will get best earnings by providing 80-90pc seat to demand ratio if you have the route to yourself.