Essentially-correct. However, the 32 aircraft quoted in the article is the right number of Amazon dry-leased aircraft in service at present (20 at ATSG, 12 at Atlas).

Remember that 1709A isn't dry-leased to Amazon; it's an Atlas-owned aircraft that is used as an operational/maintenance spare for Amazon as well as for other Atlas duties. This is why it is painted all-white. And 1399A, which went into paint in VCV a week ago (12/23/17), following a lengthier-than-normal conversion, isn't going into service for Amazon for a few months.

Amazon reserves one ATI and one Atlas 767-300 as a hot spare, one in CVG and one at ONT, but each carrier group also has maintenance spares that are used sometimes for Amazon and sometimes for other business: 255CM (767-200) at ATI, 1709A at Atlas, and 395CM at ATI. ABX has a little more slack in its fleet to cover unexpected needs than do the other two.

As to sourcing, WTFover is correct. They come from all over the place; all the 767-200s were previously-converted, and of the 767-300s, all but 1709A were specifically-converted from pax aircraft for this duty. The CAM 767-300s are less-diverse and generally have had fewer owners than the Titan ones; e.g. three retired from QANTAS and three retired from American (plus two ex-NZ that had interesting subsequent lives). Some of the ex-American aircraft being converted by CAM are older than one might expect, but are now their feedstock of choice. At least the last eight and next five CAM conversions (including ones for other customers) are retired-AA.

For the member who asked about composition of tails, there are:
Six 767-200s dry-leased to Amazon by CAM for five years and flown by ABX;
Six 767-200s dry-leased to Amazon by CAM for five years and flown by ATI (714AX, 791AX, 739AX, 761CX, 762CX, 763CX);
Eight 767-300s dry-leased to Amazon by CAM for seven years and flown by ATI (307AZ, 311AZ, 313AZ, 331AZ, 337AZ, 347AZ, 353AZ, 359AZ);
Twelve 767-300s dry leased to Amazon by Titan/Andromeda (sometimes with ownership by single-purpose entities) for ten years and flown by Atlas (1997A, 1217A, 1049A, 1229A, 1321A, 1181A, 1093A, 1361A, 1327A, 1381A, 1373A, 1409A);
Eight more 767-300s to be dry-leased to Amazon by Titan/Andromeda for ten years and flown by Atlas (1399A, 1427A, 1013A, 1511A, 1439A, 1487A, 1619A, and one more).

In each case, the carrier operating agreement is shorter than the dry lease by a couple of years. Each operating agreement is also terminable for convenience on a small number of months' notice, but the dry-lease isn't.

You all probably know this but the reason the tail numbers are not consecutive is that they are all "Prime" numbers. So, for example, in the 300 range, ATSG has used all the prime numbers except 317 and 349 (and the six prime numbers after 359). (I think 317az was reserved by CAM for a while, but no longer is, and 349az is some guy's kit aircraft.)