Sr allotment depends on how many ETL's you have. Higher volume stores have more ETL's, so they have more potential LOD's, meaning you need less senor TL's. The whole point of the Sr. TL position is to get rid of higher-waged ETL's and have TL's be able to do LOD shifts.
Some low volume stores have 3,4 or even 5 Sr. TL's because they have so few ETL's.