Just depends on your needs and what your using the PC for.
Right now I have 3X 500GB SSD's and a 1, 2, and 3TB hard drive. Along with a 1TB external drive and a hot swap bay.
For a gaming PC I would probably go for a 250 / 500GB SSD for the OS and programs with a 1TB SSD for your main games and would probably add another 1TB hard drive for lesser played games and regular storage. This would also depend on the amount of space it takes for your games.
For a work PC that you need a lot of storage and rendering about the same thing but would use the 1TB SSD for the work and you might need a larger hard drive depending on how much you store.
Also if your doing work related stuff make sure to buy something to back up your work!