Question: How many properties are maintained by the amusement park corporation that first screened the film The Sensorium in 1984?

<P0-S0> A family entertainment center (or centre), often abbreviated FEC in the entertainment industry, (also known as indoor amusement park or indoor theme park) is a small amusement park marketed towards families with small children to teenagers, and often entirely indoors or associated with a larger operation such as a theme park. <P0-S1>  They usually cater to "sub-regional markets of larger metropolitan areas." <P0-S2>  FECs are generally small compared to full-scale amusement parks, with fewer attractions, a lower per-person per-hour cost to consumers than a traditional amusement park, and not usually major tourist attractions, but sustained by an area customer base. <P0-S3>  Many are locally owned and operated, although there are a number of chains and franchises in the field. <P0-S4>  FECs are sometimes called family amusement centers, play zones, family fun centers, or simply fun centers. <P0-S5>  Some non-traditional FECs, called urban entertainment centers (UECs), with more customized and branded attractions and retail outlets, are associated with major entertainment companies and may be tourist destinations. <P0-S6>  Others, sometimes operated by Non-Profit organizations as Children's Museums or Science Centers, tend to be geared toward edutainment experiences rather than simply amusement. <P0-S7>  FECs may also be adjuncts to full-scale amusement parks. <SEP> <P1-S0> Six Flags, officially Six Flags Entertainment Corporation, is an amusement park corporation based in the United States, with properties in the US, Canada, and Mexico. <P1-S1>  It is the largest amusement park company in the world, based on the number of properties owned, and is ranked sixth in terms of attendance. <P1-S2>  In 2016, Six Flags properties hosted over 30.1 million guests. <P1-S3>  The company maintains twenty properties throughout North America including theme parks, thrill parks, water parks, and family entertainment centers. <SEP> <P2-S0>  It was produced in partnership with Landmark Entertainment. <P2-S1> The Sensorium is regarded the world's first commercial 4D film and was first screened in a Six Flags theme park in Baltimore in 1984. <SEP> <P3-S0> Shishu Park (Bengali: ঢাকা শিশু পার্ক ) is the only public sector children's amusement park in Shahbag, Dhaka, Bangladesh. <P3-S1>  Established in 1979 on 15 acre of land, it is the first children's amusement park in Bangladesh. <P3-S2>  Established as a profit making venture by Bangladesh Parjatan Corporation, the government-owned tourism promotion agency in Bangladesh, it is maintained by Dhaka City Corporation as its only entertainment park since 1983. <P3-S3>  The 12 rides of the park include including a wheel train, a merry-go-round and a number of wheel-based rides. <P3-S4>  Bangladesh Air Force donated a fighter jet in 1992. <P3-S5>  The cheapest of all entertainment parks in Dhaka, charging BDT 8.00 for entry and 6.00 for each ride, it draws about 6,000 visitors a day. <P3-S6>  During the Eid-ul-Fitr holidays the number of visitors reach up to 350 thousands. <P3-S7>  The City Corporation earns about BDT 200 million annually from the park. <P3-S8>  The park is open Monday through Thursday and Saturday from 2:00 pm to 7:00 pm. <P3-S9>  On Fridays, it is open from 2:30 pm to 7:30 pm.