You must be reading Macs autobiography.… fortifications??? 150,00 to defend Washington?
He double counted a few regiments and was several thousand high.
However, the Washington defences included the approach routes. He reckoned:
Baltimore - 7,388
Washington - 10,600 manning the fortifications
11,400 disposable troops (i.e. a field force)
4,000 new troops in NY ordered to Washington
3,500 new troops in Pa ordered to Washington
Guarding the railroads in Md: 3,359
Abercrombie's force at Manassas: 7,780 (with 12 field guns)
In the Shenandoah was Banks' 5th Corps and Blenker division of 2nd Corps: 29,715 (with 24 field guns)
Also he intended to leave Sumner and his 2nd Corps at Manassas until it was confirmed the rebels had retreated. The 1st Corps was supposed to be the amphibious turning force, and was to hold at Washington until the navy had cleared one of the rivers for them.
So yes, Washington by McClellan's account had ca. 150,000 men.
In fact, the whole force in the theatre was 191,171 present, and McClellan took the following to the Peninsula:
HQ: 73
HQ Cavalry Escort: 113
3rd Corps: 35,842
4th Corps: 34,798
Regular inf.: 4,142
Regular cav.: 2,771
= 77,674
Leaving:
1st Corps: 35,943
2nd Corps: 27,907 (inc. Blenker)
5th Corps: 23,706
Washington dist: 20,795
Alexandria dist: 1,404
Provost-Marshal's Guard: 633 (actually small dets with all units)
Arty Res: 2,906
Regular engrs: 237 (attached to 1st Corps)
The following where not AoP commands:
Railroad Bde: 4,993 (April)
Baltimore: 6,717 (30th May)
Mountain Dept: 20,432 (April, not included by McClellan)
Thus I count 145,673 left immediately to defend Washington, and after 1st and 2nd Corps left (inc. Blenker) Washington would still have 81,823 defenders, far more than the 55,000 requirement.
McClellan estimated the Confederates had ca. slightly less than 90,000 in Northern Virginia, slightly less than 20,000 at Yorktown and about 15,000 at Norfolk. In this he's actually pretty close. Whatever choices he made were not the result of false estimates of enemy strength.