Found a technical way to count by deducting available seats from total seats in the webpage code.
E01: 1280 - 1006 = 274
E02: 1507 - 842 = 665
E03: 1506 - 1012 = 494
E04: 1131 - 998 = 133
Riverside Total: 1,566
W01: 529 - 295 = 234
W02: 737 - 416 = 321
W03: 198 - 27 = 171
W04: 198 - 19 = 179
W05: 737 - 268 = 469
W06: 529 - 393 = 136
W07: 1048 - 969 = 79
W08W: 694 - 460 = 234
W08C: 700 - 459 = 241
W11C: 702 - 338 = 364
W11W: 692 - 288 = 404
W12: 1048 - 822 = 226
Jack Walker Total: 3,058
N01: 1243 - 505 = 738
N02: 828 - 238 = 590
N03: 834 - 253 = 581
N04: 828 - 272 = 556
N05: 1241 - 595 = 646
Blackburn End Total: 3,111
That gives a total figure of 7,735 seats that are unavailable.
If we knock off around 500 due to front row seats that clearly aren't taken but are classed as unavailable, then we're looking at over 7,000 sold.