When you look at raw attendance figures they can be misleading when expressing the importance of the club to its community. A different but not necessarily better guide is the percentage of the population from each clubs catchment area that regularly attends games. This is not overly scientific and there are lots of other factors coming into play. Most obvious being the recent success of each club, its facilities and competition from other sports in that area. All the same, it's interesting, the most dramatic being Leeds at the bottom, not that most clubs wouldn’t kill for those figures.
I’ve left out London because it's not fair to compare them to anyone and Catalan because they are SL’s only regional club and thus it's hard to pin down their exact catchment area.
These are not super accurate figures but close enough, I've rounded them up to the nearest 0.5%. Apologies if I've made any huge errors, I'm sure they will be corrected.  
 
 
2019 SL Figures by attendance
St Helens 				11,910
Leeds Rhinos	  			11,838
Hull FC					11,478
Wigan Warriors			        11,432
Warrington Wolves			10,970
Hull KR					8,185
Castleford Tigers			7,253
Wakefield Trinity			5,468 
Huddersfield Giants		        5,222
Salford					3,676
2019 SL Figures by the percentage of the population
						Average		Population		Percentage
Castleford Tigers			7,253		 40,210			18%
St Helens 				11,910		102,629 			11.5%
Wigan Warriors			        11,432		103,608			11%
Hull FC					11,478		130,248 (50%)		9%
Hull KR					8,185		130,248 (50%)		6.5%
Wakefield Trinity			5,468 		99,251			6%
Warrington Wolves			10,970		209,547			5%
Huddersfield Giants		        5,222	        162,949			3.5%
Salford					3,676		103,886			3.5%
Leeds Rhinos	  			11,838		789,194			1.5%