Off_Board Artillery can strike as many smoke mission as given by the scenario OOB.
But I can't see any such limitation for On-Board Artillery.
Does that mean that each mortar or artillery counter can fire smoke an unlimited times ?
Yes .
Doctrine dictated how divisional level artillery was assigned to regiments and battalions and their dual use e.g. direct fire to reduce fortifications or cover spearhead formations. In the game the WP should see more organic support as the formations are battalions whereas most of the Nato formations are companies.
"The armies of the Warsaw Pact made artillery a cornerstone of their military strategies. And the importance they placed on this type of weaponry saw them use their technology and expertise to develop a wide range of munitions including mortars, rocket launchers, missiles and large-calibre guns."
https://www.indiebound.org/book/9780995513389
"Since the late 1970s,the Soviets have developed the tank regiment (TR) into a combined arms team (tank, motorized rifle, and artillery) "
"Hence, the capability of the TR and TD to conduct largely self-supported combined arms combat has increased greatly. Large-caliber SP guns and mortars and longrange MRLs have increased the artillery available to army and front commanders. Additionally, some army-level regiments have grown to brigade size with the addition of a fourth artillery battalion. These battalions are currently expanding from 18 to 24 tubes, primarily in units opposite NATO. "
Google
Field Manual No. 100-2-3 The Soviet Army: Troops, Organization, and Equipment
FM 100-2-1, The Soviet Army: Operations and Tactics