In the iPhone app - can someone explain to me what the number on the upper left is? And how is the number determines at the start and how it countdown.. It feels like really random?

Based on the spaced repetition algorithms that skritter runs on its the number of reviews you have due. Sometimes it jumps more than 1 per review right now but i believe its being looked into - i would take it as a general estimate of how much you have due. Ideally you keep it at zero as often as you can.

There is a bug that cause it to jump up to a hundred or more at a time. When it does that I have to wait until it finish counting down to move to the next card. That too is being looked into.

There does seem to be some weirdness in the ios app - every day I now start with 2000+ to review (yup, thousand) yet within five minutes I can be down to zero. I'm fast but not that fast!!

Any idea what's going on?

There is a bug, mostly affecting users studying traditional Chinese, where all the data that was required to study those words wasn't downloaded, and so they are automatically skipped when they come up for review. The next release will fix this. In the meantime, you may want to use the site regularly to make sure you study all the words you should be studying.