Really depends on what you are designing, as well as if it is for hobby or business.
Can you describe what you will be using it for?
For education purposes, I'd say as many as you can learn. You'll never know what software the company you work for in the end will use. AutoCAD is the most common. Solidworks, and Inventor are two of the most popular used today and many other softwares are similar to them. I'd take a look at those.
tnxs its for my further education line
yeah i am mechanical engineer . nd if i wanna choose one which one is good for learning its i know SOLIDWORKS its for my education purpose projects
Catia and Pro/Engineer are the Best for Mechanical.
The Inventor, Solidedge and Solidworks are more basics.
Found this chart online and thought of this question: http://www.instructables.com/id/Which-3D-software-should-I-learn-for-3D-printing/
Your question is so actual. I am a graphic designer and have a huge experience.
There are a lot amazing softwares in the Internet, but you should pay money for using them. I decided to find something that really worth to my attention (I mean free services). And fortune smiled on me :) I want to share my findings with you - http://www.hongkiat.com/blog/25-free-3d-modelling-applications-you-should-not-miss/, https://essayservicewriter.com/, https://www.autodesk.com/solutions/3d-modeling-software.