Answer based on context:

Charleston is a popular tourist destination, with a considerable number of hotels, inns, and bed and breakfasts, numerous restaurants featuring Lowcountry cuisine and shops. Charleston is also a notable art destination, named a top-25 arts destination by AmericanStyle magazine. Commercial shipping is important to the economy. The city has two shipping terminals, owned and operated by the South Carolina Ports Authority, which are part of the fourth-largest container seaport on the East Coast and the thirteenth-largest container seaport in North America. Charleston is becoming a popular location for information technology jobs and corporations, and this sector has had the highest rate of growth between 2011 and 2012, due in large part to the Charleston Digital Corridor. In 2013, the Milken Institute ranked the Charleston region as the ninth-best performing economy in the US because of its growing IT sector. Notable companies include Blackbaud, SPARC, BoomTown, CSS, and Benefitfocus. In June 2017, the mean sales price for a home in Charleston was $351,186 and the median price was $260,000.

How many IT companies is Charleston known for?
5