If I had time to analyse large numbers this would be more accuate, but generally speaking, looking at the first 10 words in each level should be a good start for anyone who would like to develop this.
Here are the first 10 examples of single words (not phrases) that only exist at C2 in the English Vocabulary Profile (not in lower levels with other meanings and usage criteria):
Ranked by frequency in iWeb Corpus:
acre 3325 advocate 3816 allege 5567 accountable 6422 adjacent 6754 accumulation 7794 abusive 8228 adolescent 8321 abrupt 13219 affectionate 14252
Adding up their total and dividing by 10 to get an average rank of 7769. To round that to a simple figure or nearest thousand.
This would suggest that C2 users of English probably have an active vocabulary range of about 8000 words.
For C1 (same criteria):
accessory 2584 accomplish 2610 acknowledge 2722 acceptance 3903 abortion 5138 adaptation 5438 abnormal 6740 aboard 7361 addictive 8772 acquaintance 10080 / 10 = 5534.
To round up, C1 has an average of 5000 words that they actively use.
adjust 1599 acid 1801 acquire 2008 adopt 2041 accuse 3302 absence 3392 addiction 3986 adequate 3964 abstract 5362 abolish 9445 /10 = 3690. Rounded up to 4000 words. Upper-intermediate general English is a good time to learn academic English if that is their future goals.
ability 570 achieve 885 ad 1550 admit 1669 advise 1886 accurate 1888 accommodation 2707 broad 3411 accountant 5798 absent 6904 = 27268/10 = 2727 rounded up to 3000
Pre-intermediate students have about 3000 words. Which is similar to the general service list. It makes sense that it is after they have reached this point that they should start to learn more specialised vocabulary for their particular area.