Three to five battalions, approximately 1,500 to 4,000 soldiers, comprise a brigade. The size of a division varies from about 10,000 to 18,000 soldiers, and most divisions have three or more brigades of roughly equal size.
In Russia’s case, lets say it’s 15,000, so the 300,000 reservists called up (assuming they are not just flooded into the front) would be the equivalent of 20 divisions.
Just as an estimate, this is a sample list of the equipment (excluding uniforms, personal weapons and so on) which would be required to outfit a US infantry battalion:
Since it requires roughly 40 of these battalions to make up a division, multiply one of those lists by about 800 to find the pile of “stuff” witch would be required if all of them were infantry. Now start adding heavier weapons if the troops called up are artillery or armor.
The supply chain also blooms. For example, Table 7-4 (page 7-11) of the following document talks about “tons per day” of ammunition per artillery piece:
For Russian armor:
Each Russian tank regiment has around 93 tanks in three battalions, and one battalion of mechanized infantry mounted in BMP infantry fighting vehicles or IFVs (with usually around 42-48 vehicles). A motor-rifle regiment has the inverse: three battalions of mechanized infantry in BTR armored personnel carriers or BMP vehicles, and one tank battalion.
The tank army’s four units between them mustered around 16 tank battalions and 16 mechanized infantry battalions; a high ratio of tanks even by Russian standards.
In the field, these battalions were likely used to generate about two-thirds that number of ad hoc battalion-tactical groups heavily reinforced by artillery batteries and other support units.
The army also includes a brigade each dedicated to artillery (mobile BM-27 rocket and 2S19 howitzer systems), Iskander ballistic missiles, Buk medium-range air defense missiles, reconnaissance, and command-and-control.
In short, after throwing whatever stuff they could easily lay their hands on into Ukraine earlier in the war, idle minds wounder whether they have been holding back the warehouses full of modern tanks, artillery pieces and so on - not to mention uniforms, trucks and so on which will be required along with the required massive logistical support needed.
In my opinion, bringing 300,000 troops to the front as a well-oiled and well-equipped military machine is not likely and having these troops even moderately grouped before winter hits is mearly aspirational.