I also made the similar promise to myself like Alan. I'm not a packrat and I enjoy throwing the box and spare parts away when the model is completed.

I mainly focus on large WW2 German AFV (Panzer V and up) and softskin. I sold everything else that no longer interest me. Doing this actaully made me a better "focused" modeler on my topic of choice. Another advantage I saw was the reduce spending. Now I rarely purchase a model out side my subject.

There are 2 kind of models in my stash, one's scheduled to be build. The other is for collect only.

WC

My ambition is to build all the British vehicles from the very first tank (ever) to the very latest, including Artillery.

This can be pretty expensive as a lot of them e.g. FV432, Ferret, Abbot and Stalwart etc are only made by specialist companies like Accurate Armour and Firing Line.

I'm getting really tempted to try scratch building but haven't a clue how or where to start... Hmm. Time to go and haunt BARV's scratch building forum methinks.

To me, it's not really about what I can build. I know that by the time I retire, I will have far more then I could possibly build. It's more about making sure I have what I may want to build in six months or six years.
I do break up kits at times to kit-bash something so it's not uncommon to see one finished model that has had as many as five or six kits go into it (my parts box is rather big).
I freely admit to being a pack-rat.
As for what I work on at one time, I currently have five kits on the go. One is a Gunzie High Tech Ferrari 250 GTO. It's about half white metal and photo-etch so I'm taking my time. The other four are all tanks that are going into a dio so I'm doing them together so I can do all the weathering to match.