Here is what I can find on the troop strength for the early part of the 1864 Valley Campaign. Scott Patchan, Shenandoah Summer, The 1864 Valley Campaign (2007) provides brigade strengths for the troops engaged in the Second Battle of Kernstown (23-25 July 1864). Note that this will only provide Union numbers for the Army of West Virginia.
Army of West Virginia
Thoburn's Division – 4800
Wells' Brigade
Ely's Brigade
Duval's Division
Hayes Brigade – 1400
Johnson's Brigade – 1100
Mulligan's Division
Harris' Brigade – 824
Linton's Brigade – 922
Artillery – 600
Duffié's Cavalry Division
Tibbit's Cavalry Brigade – 1059
Higgin's Cavalry Brigade – 700
Averell's Cavalry Division – 1400
Schoonmaker's Cavalry Brigade
Powell's Cavalry Brigade
Army of the Valley District
Gordon's Division
Atkinson's Brigade – 1300
Terry's Brigade – 900
Yorks Brigade – 600
Wharton's Division – 2500
Smith's Brigade
Forsberg's Brigade
Patton's Brigade
Ramseur's Division
Hoffman's Brigade – 600
Johnston's Brigade – 600
Godwin's Brigade – 800
Rodes' Division – 3000
Cook's Brigade
Grimes' Brigade
Ransom's Cavalry Division
Imboden's Cavalry Brigade – 700
McCausland's Cavalry Brigade – 1400
Vaughn's Cavalry Brigade – 700
Jackson's Cavalry Brigade – 700
Johnson's Cavalry Brigade – 1500
Artillery – 900
Then going on to Scott Patchan, The Last Battle of Winchester, Phil Sheridan, Jubal Early and the Shenandoah Valley Campaign August 7 – September 19, 1865 (2013) we find the following for the Union at three different times – pre-Aug 1864 / Aug 31 / Sept10:
Army of the Shenandoah
Infantry
VI Corps – 11144 / 11333 / 12028
XIX Corps – 5812* / 12068 / 12150
* plus one brigade detached as wagon guards – 1500
Army of West Virginia – 6500 / 7029 / 6834
Cavalry
Merritt's Division – 3500 / 3434 / 3424
Averell's Division – not present / not present / 2500
Duffié's Division – 800 / not present / not present
Wilson's Division – not present / 2957 / 2702
Artillery – 1405 / 2025 / 1535
Rebel strength is given for four dates – Aug 20 / Aug 31 / Sept 10 / Sept 30:
Army of the Valley District
Infantry
Gordon's Division – 2823 / 2883 / 2690 / 1750
Rodes' Division – 3386 / 3305 / 3244 / 2345
Ramseur Division – 2060 / 2045 / 2010 / 1449
Wharton's Division – 2000 / 2100 / 2172 / 1071
Kershaw's Division – 3500 / 3445 / not present / not present
Cavalry
Lomax's Division – 4091 / 3900 / 3215 / 2000
Fitz Lee's Division – 1500 / 1400 / 1370 / 1000
Artillery – 1000 / 1000 / 813 / 720
One more minor detail can be added from William Miller, Decision at Tom's Brook, George Custer, Thomas Rosser, and the Joy of the Fight (2016) where it is mentioned that when Rosser's Laurel Brigade arrived in the Valley on 5 October it numbered 600 men.
I hope this is of some help.