Back during the Second World War it was a rule of thumb that major battles consumed about a battalion's worth of combat troops a day. Exhausted divisions (or infantry regiments) would be withdrawn from the front and refreshed in a safe area. Sometimes entire divisions were reduced to a skeleton force and not merely refreshed, but rebuilt over months. The armies of WW2 became accustomed to the need to supply fresh bodies and new hardware to the front at appalling rates.
Later on, in 1967 and 1973 and again in 1991, clashes of all-motorised, armor-heavy forces destroyed astonishing quantities of tanks within a few days. Hundreds of tanks were destroyed daily (on average), exceeding even the mass destruction of tanks at Kursk 1943 on many days. This rapid rate of attrition freaked out NATO and presumably also Warsaw Pact army officer corps, but European NATO never really built up appropriate reserves of tanks, likening itself rather to the winning side of these tank destruction events. The U.S.Army instead kept building excess M1 Abrams, keeping thousands of older M1 Abrams models in storage at home.
Fast forward to current affairs:
Brigades and (reinforced) battalion battlegroups are the dominant formation for ground forces employment for conventional warfare. Brigades can be expected to have one or at most two tank battalions, reinforced battalion battlegroups one tank company or battalion. That's anything from about a dozen to about a hundred (main battle) tanks. In other words; one inept or unlucky move and the entire battlegroup may be very weak on tanks if not tankless (tank brigades would rather run out of infantry first).
Obviously, those formations would need to be able (prepared) to switch to a different mode of combat until the critical losses (this may be infantry, artillery or other specialist assets as well) would be replenished. Ideally, this would happen safe from opposing forces and offer time for an exercise to smoothen out the cooperation of old and new troops (to reduce friction) and give the leaders an idea of the capabilities of the new replacements.
There may be some infantry reserves (mostly troops who left the service only recently or who served in a different branch after their infantry service), but there are few if any tank reserves in European armies. The excessive focus on peacetime operation instead of on wartime strength (past the first two weeks) can be blamed for this. Germany, for example, exported most of its Leopard 2 tanks. Almost all of them are in service in NATO allies and thus most exports didn't weaken the alliance, but the German tank forces count their reserve tanks only in the dozens, and to date most of those weren't modernised for about two decades.
The situation isn't much different in the UK, France, Italy and so on. The Western perspective on the 1991 Gulf War is dominant - the experience of marginal losses - rather than the Iraqi perspective of seeing one's forces melting away in mere days.
Military history shows that armies tend to blunder early in a war if they weren't at war for a long time (and "a long time" may be as short as five years - see the U.S.Army's experience in Korea during 1950!). It's thus in my opinion wasteful to focus on peacetime strengths at the expense of wartime strengths. A country tends to overspend on support services if it focuses on a well-rounded peacetime force instead of on wartime (mobilised) force, which naturally has a smaller share of support services because attrition rates of and thus reserves for combat troops would be higher.
In other words; to have a tank battalion and a tank battalion's worth of tank replacements as well as two companies worth of tank crew replacements is much cheaper than to maintain two tank battalions, but in the event of a war it might be almost as useful.
This leads to two points which I covered many times already: Military expenses should be expenses for deterrence (Would potential aggressors agree that the reserves are important?) and the problem of a quick and short aggression (coup de main & fait accompli).
A quick and short aggression would be smaller and more limited than an all-out yet still "conventional" WW3. You would need relatively few but very high readiness/rapid deployable or in-theatre forces to deter a quick and short aggression. Meanwhile, you would need much more military power to deter a larger war.
An obvious compromise comes to mind: Set up active high readiness peacetime forces of the size and located as required to deter a quick & short conflict, and add the extra strength required to deter a larger continental conflict in great part with reserves (mobilisation formations and replacements).
As usual, I don't exactly feel that the mainstream does so. I think mainstream military policy is neither doing a good (efficient) job at providing deterrence to a quick & short conflict by having high readiness early defence forces nor does it exploit the cost-savings potential of reserves properly or even only signal to the world that the army could last and remain highly effective for many weeks even in a peer conflict.
There's no trace of this in army doctrines as far as I know (and that's more than I should be able to know). Field manuals simply have no chapter on "how to conserve strength while avoiding defeat". There are chapters on delaying actions, but only rudimentary ones that usually fit better to 1942 than to 2016.
Mainstream / establishment military policies rather seem to serve the bureaucracy itself: Strength on paper gets neglected, while the active force with its very real job for officers gets all the attention. You cannot derive much prestige from reserves either - or else everyone would rate the Swiss militia army much higher than the German army.
This is again one of the topics where I'm missing the political master's intervention in favour of a more reasonable, nation-serving policy and less bureaucratic self-service.