Yup, there are five Segmentums: Solar, Pacificus, Obscurus, Ultima, Tempestus.
A Sector is an volume of space roughly equivalent to 8 million cubic light years. The Sector itself is composed of subsectors (themselves centred around heavily populated star systems), and intervening wilderness space.
The estimated number of sectors differs based on how you try and go about it. Unlike subsectors, there is no space between neighbouring sectors. Assuming the Imperium takes up 1/2 the volume of the galaxy, then there are 2.4375 million sectors. Most of these won't be inhabited, contain subsectors, or have a defending fleet other than the very occasional patrol.
Alternatively, if you assume something like the Scarus Sector is typical, it consists of 35 worlds. Distributed over the Imperium, that makes 28,571 Sectors.