The number of satellite signals a GPSr can track simultaneously varies by model. A minimum of four satellite signals are required to calculate your position. Many of Garmins GPS only navigators (pre-GLONASS) were equipped with 12 channel receivers, and were able to track 12 satellite signals simultaneously. Newer Multi-GNSS models are equipped with more channels to accommodate the additional satellite constellations they can track. This does not mean the GPSr uses all received signals to calculate your position, however. Each device will choose the best signals from those available for this task.
If you count the number of available signal bar spaces on your Montana 7x0 Satellite page display, you will see there is room for 12 on each side. The left side is always GPS, the right side is either Galileo or GLONASS, or none, as configured.
As you point out, Satellite ID 49 is displayed in the 'SkyView' above, but not listed in the signal bars below. This is because Satellite ID 49 is an EGNOS satellite that provides differentially corrected data for all other satellites in the system, and is not used like the others for position calculation. The Satellite signal bars with the 'D' displayed are 'Differentially Corrected' using data from, in this case, satellite 49.
Please see
GPSrChive > GNSS > SBAS > EGNOS for additional satellite constellation information.
Please see
GPSrChive > Montana 7x0 > Main Menu > Satellite for additional Montana 7x0 Satellite page details.