What is the number of the Interstate loop around Houston, Texas?

There are two ring roads around Houston. The inner road is the East and West Freeways which each have a north and south loop each forming an inner loop around the city which is route 610; there is also an outer ring road. This is the Sam Houston freeway called Route 6 and runs around the entire city in the shape of a square.

https://www.hctra.org/tollroads/

The only "Interstate" designated loop around Houston is 610. The larger ring around Houston is the Sam Houston Tollway and Beltway 8.

Loop 610 is a 38 mile (61 km) open freeway with no traffic lights. It also has several uninterrupted interchanges with major highways in the area.

Beltway 8 is designated as a State Highway and is sometimes referred to as the Sam Houston Parkway. It basically forms the frontage roads for the Sam Houston Tollway. Beltway 8 was built first, then the Tollway filled in the space between the frontage roads.

Beltway 8 is 88 miles long (142 km) and has many stop lights as it intersects many other smaller roads along its path. The Sam Houston Tollway has no traffic lights and several large uninterrupted interchanges with other major highways in the Houston area. With the exception of a few toll plazas, the Sam Houston Tollway is an open highway with no interruptions.

An even larger ring known as the Grand Parkway (or State Highway 99) is currently in various stages of completion and construction around the Houston metropolitan area.

https://en.wikipedia.org/wiki/Interstate_Highway_610_(Texas)
https://en.wikipedia.org/wiki/Interstate_Highway_610_%28Texas%29
https://en.wikipedia.org/wiki/Beltway_8
https://en.wikipedia.org/wiki/Texas_State_Highway_99