Question: how often do panzers spawn origins?
Answer: Panzers come every 4 rounds. 2 soldats per 4 rounds is the maximum after round 16 (only 8th and 12th rounds have 1 soldat). If you kill all the zombies and haven't killed the soldat(s) then a new round will begin. Panzer(s) will only start to spawn if you open No Man's Land.