Russia would not actually fight with Iran but will provide various military systems.
CEP is used to measure missile accuracy. CEP (Circular Error Probable) is defined as the radius of the circle in which 50% of the fired missiles land. The smaller it is, the better the accuracy of the missile.
The latest version of China’s DF-21 has a CEP about 40 meters which allow accurate targeting of small complexes or large buildings. The CEP of the American made Hellfire is as low as 4 meters and enabling precision strikes on small buildings or even tanks.
If you were trying to hit a 100 meter by 100-meter complex. Forty missiles with a CEP of 150 meters would have a 90 % chance at hitting the complex. Three DF-21 missiles would have achieved the same result. The US missiles would in general do not miss such a large target.
The R-11M ‘Scud A’ entered into service in 1955. The ‘Scud A’ was 10.3 m long and had a diameter of 0.88 m. The missile had a range of 190 km and was accurate to about three km CEP (about 2 miles).
The R-17 ‘Scud B’ was an upgrade over the ‘Scud A’ that became operational in 1962. The SCUD B has a range of 300 km with accuracy of 450 meters CEP.
Over 600 ‘Scud B’ and North Korean ‘Scud B’ variants were fired by Iran and Iraq between 1980 and 1988. Over 2,000 ‘Scud B,’ and possibly a small number of ‘Scud C’ missiles, are thought to have been used in Afghanistan.
In 2010, Iran developed an improved version of the Shahab-2 called the “Qiam-1,” with an 800 km range, better accuracy, and a separating re-entry vehicle.
The Shahab-3 is a medium-range ballistic missile (MRBM) developed by Iran and based on the North Korean Nodong. The Shabab-3 has an accuracy of 140 meters CEP, the Shahab-3 missile is primarily effective against large, soft targets (like cities).
US Air Force National Air and Space Intelligence Center estimates that as of June 2017 fewer than 50 Shabab-3 launchers were operationally deployed.
Iran has the largest ballistic missile force for Middle Eastern countries. Iran has deployed to military units about 100 road-mobile launchers for SRBMs perhaps half as many road-mobile launchers for MRBMs, and hundreds of associated missiles (perhaps over 1000).
The missiles have unitary high-explosive and/or submunition warheads payloads ranging in size from 250 to 1800 kg.
The Shahab-3B has a fast fueling capability and might be accurate to a CEP of 30-50 meters. In any conflict with between Iran and the USA or Israel, Iran would need to move and hide missiles and would need to rapidly shoot them before they were destroyed by US planes and missiles.