If the "level" numbers were to happen to have a decimal point in them, the maximum level 55, read as meaning 10^5.5=316,228 would be quite close to the number of entries in the 2nd edition of the OED, which is 291,500.
Time for today's attempt at guessing my way to the big one...
(NB. I'm sure it ISN'T what I was suggesting above, as it has been retrying some words I had already got right once. It has now stopped testing MY vocabulary and is testing how well I have learnt ITS vocabulary. I learn quick.)
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"Striking up conversations with strangers is an autistic person's version of extreme sports."
Kamran Nazeer