Wanted to quickly address this one as well, this time looking at brigades instead of regiments. I'm fully aware that Jackson straggled badly.Even after Jackson’s arrival, Lee has little more than half as many men as the AotP.
Jackson was in line by noon; at this point the Army of Northern Virginia is missing Walker, McLaws, Anderson and AP Hill.
The AoNV has a total of 39 brigades: Walker (2), McLaws (4), Anderson (6) and AP Hill (5) are missing from it, while DR Jones (6), Hood (2), Evans (1), Ewell (4), Jackson (4) and DH Hill (5) are in line, giving him 22 brigades with another 2 able to arrive by the end of the day.
At this time McClellan has 1st Corps (10), 2nd Corps (9) and Sykes (2.5) in line, with 9th Corps (8) and 12th Corps (6) en route but not arrived yet (i.e. basically in the same position as Walker). This means that in brigade terms McClellan has 21.5 arrived and another 14 able to arrive by the end of the day (but only for a head-on attack).
Even if we assume Jackson has straggled so badly that his four brigades temporarily only count as one, McClellan has rough parity in brigade count. For Lee to be outnumbered about 2:1 before noon on the 19th then the average Union brigade has to be twice the size of the average Confederate brigade.
This is simply not credible. Both armies' brigades average about 4.5 regiments each (rough figure, more than 4 but less than 5). Some of the Union regiments are brand new, but you'd need every single Union regiment to be brand new to get the near 2:1 ratio.