I´m thinking to travel to Buenos Aires, but I don´t know how many days it´s necessary to visit the cool and beautiful places in the city.
No less than 5 and it really depends on what your interests are. Buenos Aires doesn't really have any knock-out tourist attractions and what's here can be covered quickly. It's the life of the city itself - eating out, going out, meeting people, walking the parks and the port, staying out until 8 in the morning - that will either grab you or it won't. It grabbed me a year and a half ago and I've never left.
If the weather is still good, don't miss a day trip up the coast to Tigre in the Delta.
One solid piece of advice: Stay at a hostel so you can meet people. It's the only way. almost 8 years ago