I will be applying for a US visa to attend a scientific conference, also I plan to stay a week more to travel around. Should I apply for visa category B1/b2 (businesss and tourism)?
First of all, you only have one status in the US at a time. Even if you travel to the US on a B1/B2 visa, you will still be admitted on either B1 status or B2 status.
If you enter the US on B1 status for your conference, you do not need to apply for Change of Status to B2 status to travel for personal purposes inside the US for the remainder of your period of stay. From this USCIS page:
You do not need to apply to change your nonimmigrant status if you were admitted into the United States for business reasons (B-1 visa category ) and you wish to remain in the United States for pleasure before your authorized stay expires.
So being admitted on B1 status is sufficient for your purposes, assuming you are admitted for long enough to cover your intended travel.
For the purpose of being admitted on B1 status, you can travel to the US on either a B1 or B1/B2 visa. Either will work.
Most people who apply for B1 or B2 visas will still get B1/B2 visas. So you could apply for just a B1 visa if you don't have any plans for a separate personal visit in the future, and most likely you will still get a B1/B2 visa.