Why is the new movie named Ocean's 8?

Because there are eight con-women in this movie as there were 11, 12 and 13 con-men in the Ocean's trilogy...

Based on Wikipedia:

Ocean's Eleven:

1. George Clooney as Danny Ocean
2. Brad Pitt as Rusty Ryan
3. Matt Damon as Linus Caldwell
4. Don Cheadle as Basher Tarr
5. Bernie Mac as Frank Catton
6. Scott Caan as Turk Malloy
7. Casey Affleck as Virgil Malloy
8. Carl Reiner as Saul Bloom
9. Elliott Gould as Reuben Tishkoff
10. Eddie Jemison as Livingston Dell
11. Shaobo Qin as "The Amazing" Yen

Ocean's Twelve:

12. Julia Roberts as Tess Ocean

Ocean's Thirteen: (Replacing Julia Roberts as the 12th member)

12. Andy García as Terry Benedict

13. Eddie Izzard as Roman Nagel

Ocean's eight:

Danny Ocean's estranged sister, Debbie, attempts to pull off the heist of the century at New York City's star-studded annual Met Gala.
Her first stop is to assemble the perfect crew:
Lou, Rose, Eight Ball, Tammy, Amita, and Constance.

That's seven members and the 8th member is apparently secret as in usual Ocean's fashion...

There's no doubt that Nikhil has the right answer for the question as posed, but it might be worth thinking about why they went with 8 and not 14 (as would have been next in the series), or 10 (just before 11) or any other number.

I personally think they were reaching the limit for how many main characters they could have. When they wanted an all female cast, they needed an all new cast (it would probably have been confusing to put Julia Roberts in) and that means they need time to tell some backstories, but they also want time to tell a story. It's a lot easier to squeeze in 8 than 14. Also it's easier for the audience to separate 8 characters and remember what each one's skills are (I know that I struggled the first time I watched Ocean's 11).

By going back to 8 instead of just 10, they still have enough that's it's a large enough gang to pull off a spectular heist and they leave room for some (2, i.e. a new trilogy) successors with consecutive numbers. - If the movie doesn't flop big time, my money is on us getting "Ocean's 9" within a couple of years.

Ahem... 8th March is International Woman's Day. It can't be merely a coincidence.

International Women's Day (IWD) is celebrated on March 8 every year. It commemorates the movement for women's rights.

[...] March 8 was suggested by the 1910 International Woman's Conference to become an "International Woman's Day." [..] it was adopted in 1975 by the United Nations.

By Henrik's logic, Ocean's 7 would have worked equally as well, there's one less name to remember, and the number seven recurs in several movies but the producers (I assume) chose 8. They could have gone for 6 but that number is not generally considered lucky in the Occident whereas the number 8, in China, is considered auspicious.

No. 8 has long been regarded as the luckiest number in Chinese culture. With pronunciation of 'Ba' in Chinese, no. 8 sounds similar to the word ? 'Fa', which means to make a fortune

from the same website

The popularity of 8 was obvious in relation to the Beijing Olympic Games which commenced at exactly eight o’clock eight minutes on the eighth day of the eighth month in 2008. When people choose telephone numbers, mobile numbers, house numbers, car identification numbers and important dates, 8 is usually the first choice.

Another coincidence? Could be.