Here is a web page which gives some information about oil consumption.
Excessive Oil Consumption Isn't Normal - Consumer Reports
It's written (I suspect) for the American market; a 'quart' of oil will be around 1.1 litres (a litre of water's a pint and three quarters!).
In the UK the need to put one or two pints of oil into a four year old engine between servicing/oil changes (10K to 15K klicks and 12 months) would not be 'excessive'.
You will be using your car where it is much hotter and this will probably cause slightly higher oil consumption. Also, if you are 'blasting off' a lot , which is always a temptation in an SLK, the consumption of oil, like petrol, will rise.
If you acquired the car just 5 months ago and it is still under warranty you might wish to speak to the seller about your concerns, just to set your mind at rest.
However, the most important thing is to make sure you regularly check the oil level and keep it topped up to (but NEVER over) the mark on your dip stick. Half way is good. The car will (usually) tell you if the level falls too low but it should be a point of honour to never allow that to happen! You can also take the occasional look in the rear view mirror. Take heart if you can see the envious faces behind you and, more importantly, there is no smoke from the exhaust of your car!!
Enjoy!!