You might have 12 rounds (2x6) on a tracked MLRS and 6 on a wheeled Himars. However, I think the practical comparison is between BM-30 and A-100 and tracked MLRS as wheeled Himars is simply a light version for expeditionary forces.
Easy way to compansate for fewer missiles per wheeled vehicle: put more vehicles in a battery. Also, do not under estimate the advantage of the loading system employed in MLRS and Himars: they can independently pick up six packs preposition somewhere in the field and load them without assistance. So, they can shoot, quickly reposition and reload on the way. Quick reloading - in a static position - means a higher sustained/continuous rate of fire. Shoot and scoot, aided by rapid in/out of action times and quick launcher laying, translates to survivability, in environments where launched are monitored by arty/rocket locating radars and where counter battery fire can be expected.
This is copied in the Chinese SR-5 MRL
It can fire ATACMS from single round pod. ATACMS is a ballistic missile. That leaves the launcher vehicle unchanged.
Better comparison would be Chinese SR-5
http://www.military-today.com/artillery/sr5.htm
LIMAWS(R) will provide the Royal Artillery's fire support to the British Army's light and rapid reaction forces.
The chassis selected for the LIMAWS(R) is a 6x4 Supacat chassis.
The vehicle is designed and built with four lifting points so it can be carried underslung by a Chinook CH-47 helicopter.
The total vehicle weight, including a 2.3t rocket pod, is approximately 9t.
LIMAWS(R) is fitted with a computerised fire control system supplied by Lockheed Martin Missile and Fire Control Systems and which is of proven design and is also installed on HIMARS vehicles and on the tracked version of MLRS.
LIMAWS(R) is fitted with a new lightweight launcher. The launcher is pivoted at the rear of the vehicle chassis. The elevation system is hydraulically powered. There is no traverse turntable as in MLRS but the boom reloading system is of similar design. In a typical firing mission, the target data is transmitted to LIMAWS(R) from a battlefield command post.
http://www.army-technology.com/projects/limaws
The HIMARS vehicle weighs approximately 24,000lb (
10,886kg) compared to more than 44,000lb (
19,958kg) for the MLRS M270 launcher. HIMARS retains the same self-loading and autonomous features installed on the MLRS.
HIMARS is operated by a crew of three - driver, gunner and section chief - but
the computer-based fire control system enables a crew of two or even a single soldier to load and unload the system. The fire control system includes video, keyboard control, a gigabyte of programme storage and global positioning system. The fire control computer allows firing missions to be carried out in automatic or manual mode.
In a typical mission, a command and control post would transmit the selected target data via a secure data link to the HIMARS on-board launch computer. The computer then aims the launcher and provides prompt signals to the crew to arm and fire a pre-selected number of rounds.
The launcher can aim at a target in just 16 seconds. It is possible for the crew to select preprogrammed multiple mission sequences which have been stored in the computer.
Two advanced medium-range air-to-air missiles (AMRAAM) were successfully fired by the HIMARS launchers in March 2009....
http://www.army-technology.com/projects/himars/
By comparison:
A loaded BM-30 launch vehicle weighs 43.7 t.
An A-100 launch vehicle weighs 45 t.
The SR-5 launch vehicle weighs 25 t