This used to be my favourite subject way back when, partly because at the time it was one of the few areas you could find decent organisational information on. I still think the authorised strengths are the best place to start, as these demonstrate what various armies thought was needed to both function and absorb losses.
British and Canadian Rifle Section, 1943-45
Corporal (Sten); Rifle Group (6 men, each with rifle); Gun Group (3 men, Lance-corporal with rifle, No.1 with Bren gun and No.2 with rifle).
* The 1944 infantry manual also offers the basic load of ammunition for the Section. 5 magazines for the Sten, 50 rounds per rifle and 1000 rounds for the Bren. The latter was carried in 25 magazines (two per riflemen and four per Gun Group member, plus one in Bren) for 700 rounds, and 50 per man in the Rifle Group not in magazines.
US Rifle Squad, 1943-45
Sergeant (later Staff Sergeant) (M1 rifle); 7 riflemen (two designated as scout, all with M1 rifle); Corporal (later Sergeant) (M1 rifle); BAR group (Automatic rifleman with BAR, assistant AR man and ammunition bearer, both with M1 rifles).
* I've not got hold of the Equipment parts of the Tables of Organization and Equipment for the Infantry Battalion, but understand from those who have that three rifles were to be fitted with grenade launchers. Despite looking recently I've not seen a suggested personal ammunition load but the rifleman belt carried 80 rounds and bandoliers 48 each. BAR magazines pose some problems; the gunner was equipped with a belt that could hold 12 magazines, plus one in the BAR, but exactly how many the other two team members were anticipated as carrying I've never seen. As a comparison USMC manuals give 13 for their AR man and 12 for his assistant under the 1944-45 organisations.
German Gruppe/Squad, late 1943 to 1944
NCO (MP); 8 men equipped with 1 MP, 1 LMG ( plus pistol for the operator) and 6 rifles.
* The KStN do not identify who was to be armed with what weapon within the Squad other than its leader. Instead there is an overall summary for the Platoon, which you divide by three to get the Squad armament. Of the six rifles one was to be fitted with a telescopic sight, one was to be self-loading and one was to have a grenade launcher attachment. The previous Squad consisted of a three man team for the LMG (gunner and assistant both with pistols, ammunition bearer with rifle) and six riflemen, one of whom was the second to the Squad leader. When the Squad dropped to nine men it is generally assumed the LMG team was reduced to two, but I don't know if that is something that is officially stated somewhere.
As for ammunition, the MP40 belt carried six magazines, the rifleman belt could hold 60 rounds. The Squad manual for the 10-man Squad reckoned the LMG numbers carried between them 5 drums of 50 rounds each and three ammunition boxes of 300 rounds each, total 1150 rounds. There's a thread of mine somewhere on the small arms section as to whether these latter were loose or belted.
Red Army Squad, full strength version, 1943-45
Sergeant (rifle, later SMG), Junior Sergeant (LMG), assistant (Rifle), 6 riflemen (5 rifles and 1 SMG).
* There were two versions of Rifle Squad in the Rifle Platoon, the other was as above but deleted two riflemen and added a second LMG team. Also during 1943 the third Platoon in each Rifle Company was to exchange its rifles for SMGs. In theory all the rifles were supposed to be semi-automatic models, but it's generally accepted that bolt action rifles predominated.
The problem of comparing book strengths to actual is that the latter is so hard to quantify. Most Rifle Sections or Squads could continue to function if reduced to around six men, because they still have the ability to perform their own fire and movement to some extent. Once you lose anything from the trio of leadership, ability to project automatic fire and provide riflemen for close defence or close assault, a Squad is not truly capable of performing its intended role. As long as the LMG can be sustained it can certainly defend, but without riflemen it cannot cover much frontage. Sidney Jary noted in his 18 Platoon memoir that he felt most German Squads were indeed just keeping the LMGs firing.
Beating the same ground again to an extent I know, but regarding captured weapons I have come to wonder whether their significance and prevalence has been much over estimated. As a theoretical example, in a neat formation such as First Canadian Army, with two Infantry and one Armoured Divisions, you have over 750 Rifle Sections across 21 Infantry Battalions at full muster. If you want to write that figure down due to wastage to something closer to say 600, that's still a fair number. So how many of those Rifle Sections are actually going to incorporate a captured MG34 or MG42, or more exotic German Army weapon? I'd argue it has to be at least a quarter before it can be considered a common eventuality and I don't know how on earth it can be measured. All the problems common to bringing unsupported weapons into the supply and maintenance requirements of a Rifle Company would suggest it would be difficult to sustain trophy weapons for very long in the field, as you're having to scrounge ammunition, spares and possibly lubricants just to keep them running.