Wonders of World Engineering was completed in 53 weekly parts. You can see all the covers listed here for Volume 1 and here for Volume 2.
How many colour plates were there?
25 in total, 11 in volume 1 and 14 in volume 2. A complete list (with links to the plates) can be seen here.
How many photogravure supplements were there?
29 in total, 17 in volume 1 and 12 in volume 2. A complete list (with links to the plates) can be seen here.
How can I obtain copies of individual issues or the bound volumes?
Try Ebay, where individual copies appear regularly, and you can sometimes bid for complete (or near complete) sets.
Bound copies are easier to obtain as most collectors of the original series had their individual copies bound by the publishers. Further details of binding arrangements are posted here.
Other useful sources include eBid, Amazon and Abebooks. Or try charity sources like Oxfam.
How much is my set of Wonders of World Engineering worth?
Perhaps not as much as you might think. Individual copies start at 99p and upwards on Ebay. Bound sets can start at £10-£15 and upwards. Remember that condition will also affect the price. For individual issues, parts 1 and 50 are usually the most battered for the simple reason if the set was kept in a pile, these copies were top and bottom. Covers can become creased (which can mar the colour illustration), whilst the staples often rust over time (which can cause further staining or discolouration of the pages).
For the bound sets, the most common problem is damage to the spine, which often become discoloured over time. The binding covers can also become detached from the contents. You may want to check that all the plates (colour and photogravure) are intact. Also, note there were several versions of the bindings used, the most common of which are fully described on this page.