Connecting LED light strips in parallel
2.4 W per 20&quot; = 0.12 W per inch.
7 shelves × 48&quot; = 336&quot;
336&quot; × 0.12 W/inch = 40 W.
From P = VI we can calculate the required current = 40 / 12 = 3.3 A.
You need a 12 V, 4 or 5 A PSU with a 0 to 12 V voltage adjust capability. Usually for LEDs the dimming will be done by pulse-width modulation (PWM) which switches them fully on and fully off at a high enough frequency that the eye doesn't notice any flicker.

Figure 1. PWM signal transitioning from high pulse width (75%) to low (25%) and back again. Note amplitude remains constant. Image source and more reading on Dimmable mains power supplies.