Not as much as you think! When I first started out in the bed linen business, sales representatives referred to cotton bed sheets and duvet covers as five year and ten year cotton. This referred to the life expectancy of the different qualities. Both qualities were still classed as regular cotton. In fact even the five year cotton that they were selling at the time, I still use now over thirty years later so perhaps they were selling themselves and their product a little short. However the ten year cotton sheets did have that extra bit of body in it and a better finish, and I will still reach for those sheets unless I’m stuck. The five year quality had a light quality about them that I never really liked. I only bought them at the time to check the quality out and see what the sales people were talking about The five year quality never made it to the shop shelf.
A lot of suppliers will tell you that the quality of the cotton thread and the quality of the weave would be of more concern than the thread count. Also some people like a sturdy sheet and others are into the light silky sheets. For those with sensitive skin and particularly those with allergies, softness and pliability might be what works best, so check out the feel of what you have at home and decide on your favorite type of cotton for your sheets and duvet covers.