Air carrier pilot reported a lack of adequate taxiway markings from taxiway S to Spot 20 at MIA airport.

Narrative

Upon arrival at MIA they gave us parking at gate X since we were arriving almost 3 hours late and the usual positions were no longer available. The control makes us roll along the taxiways T7/S/V/Q/S to enter the gate through spot 20. When we arrived on taxiway Q to the taxiway S we realized that there was no yellow line to enter spot 20. We also see that to continue rolling through Q to enter through T or; failing that; to have entered through U (both have yellow lines) would have been impossible since there was a wingspan restriction. By taxiway P from U to the east; and in this case there were aircraft in the positions involved so no special authorization could be requested either. Given all that and seeing that there was no obstacle that prevented us from entering the position; we continued through S; spot 20 and our gate despite not having the yellow line between S and Spot 20. This situation caught our attention and we asked if that position was really properly assigned for us to operate.