Long post coming, fair warning.
Fox published 14 comics in 1939 (Wonder/Wonderworld 1-8, Mystery Men 1-5, Fantastic 1). The total number of published books was 3,456,422, for an average of 246,887 per issue.
Fox published 67 comics in 1940 (Fantastic 2-13, Mystery Men 6-17, Science 1-8, Wonderworld 9-20, Weird 1-9, Blue Beetle 1-3, Flame 1-3, Samson 1-2, Rex Dexter 1, Green Mask 1-3, Big 3 1-2). The total number of published books was 13,125,833, for an average of 195,908 per issue.
Taken together for 1939-1940 the average is 204,719 per issue.
The monthly breakdowns are as follows:
Yr/mo (# of comics published) avg published per issue
1939
J 0
F 0
M 0
A 0
M 1 --?
J 1 --?
J (1) 227,533
A (2) 209,298
S (2) 314,264
O (2) 341,849
N (2) 328,682
D (3) 296,900
1940
J (4) 247,543
F (4) 343,057
M (4) 280,819
A (5) 227,295
M (6) 235,478
J (7) 123,268
J (6) 194,041
A (5) 193,088
S (10) 112,082
O (4) 239,998
N (4) 252,132
D (8) 125,985
These numbers are taken from those cited previously in this thread. I feel confident that the dates given correspond to the month given in the indicia because the doubling in numbers between July & Aug 1939 match the addition of Mystery Men to Wonderworld. I also assumed that a Spring, Summer, Fall, or Winter issue was published in Mar, Jun, Sept, or Dec -- which I believe is correct based off of the way they transitioned from quarterly to bimonthly. However, I can't verify this as I can't find any of those titles with arrival dates. If anyone has a house ad or arrival date for Samson, Big 3, Rex Dexter, Flame, or Green Mask to confirm this would be helpful.
Surprisingly, few patterns emerge from this. Conclusions that I can speculate are:
1) Fox seemed conservative when issuing new titles. The delta between a month where a new #1 was published and the previous month is significantly less than the average per issue published in the prior month. e.g. Aug 1939 Wonderworld 3 was 227,533 copies. Sept 1939 when Mystery Men 1 was added to Wonderworld 4, Fox published a total of 418,597 books, a difference of 191,064. This pattern holds true for all first issues, to the degree I can evaluate. Conventional history says that GA first issues were often published in higher than usual numbers to get the title out there, then the 2nd & 3rd issues were published in lower numbers until the returns came back on the first issue to see how sales were. If this is true, Victor Fox went against the grain.
2) It may be the case that the total amount of issues Fox could publish in a given month was capped by the publisher, finances, or other reasons. For months where 4-6 issues were published the numbers were generally in the 1M range. In two months where 8 and 10 issues were published, the total numbers were 1.1M and 1.0M. Sept of 1940 there were 10 issues, three of which were first issues, and he only published 1M books. This could be him being conservative with first issues again, or he may not have had the ability or funds to publish more than a million books or so.
3) Over time the average numbers trended down, although they varied widely. I'd be interested to see what happened in 1941.