The short is answer is “None at all” (unless you have hardwood decking). That is to say, you don’t have to do anything if you don’t want to. The timber is guaranteed by the suppliers/board manufactures for 15 years against rot and insect infestation. A properly constructed deck, built clear of the soil, with plenty of room for air to circulate, will last many more years than this.
However, any timber, hardwood or softwood, when exposed to the elements, will eventually turn a silver grey colour if untreated. It can also develop small splits and cracks when it is really dry. This does not in any way detract from the strength or integrity of the deck but may not be aesthetically pleasing. So, to maintain it’s appearance I usually recommend giving it a coat of decking oil once a year. There are various proprietary brands on the market, tinted as well as clear.
The only other thing is to keep it clean, especially if it doesn’t get much sun, and starts to get a green algal coating. This can be slippery when wet. Again, there are various products available but I find a pressure washer works well and doesn’t do any harm.