Going by the averages below I think what you've been "advised" is wrong or either people have finally stopped going to watch Big Sams master classes in tactics or ST holders who haven't been able to make it. The only seats that I've seen empty are those terrible obstructed view seats but even they sell a decent amount to people willing to sit behind a post.
Sat 12 Aug
Stoke City (H) 1 - 0 Rooney - Attendance - 39,045
Sat 9 Sep
Tottenham Hotspur (H) 0 - 3 - Attendance - 38,835
Sat 23 Sep
Bournemouth (H) 2 - 1 Niasse 2 - Attendance - 38,133
Sun 1 Oct
Burnley (H) 0 - 1 - Attendance -38,448
Sun 22 Oct
Arsenal (H) 2 - 5 Rooney, Niasse - Attendance -39,189
Sun 5 Nov
Watford (H) 3 - 2 Niasse, Calvert-Lewin, Baines (pen) - Attendance -38,609
Wed 29 Nov
West Ham United (H) 4 - 0 Rooney 3, Williams - Attendance -38,242
Sat 2 Dec
Huddersfield Town (H) 2 - 0 Sigurdsson, Calvert-Lewin - Attendance - 39,167
Sat 16 Dec
Swansea City (H) 3 - 1 Calvert-Lewin, Sigurdsson, Rooney (pen) - Attendance - 37,580
Sat 23 Dec
Chelsea (H) 0 - 0 - Attendance -39,191
Mon 1 Jan
Manchester United (H) 0 - 2 - Attendance -39,188
Sat 20 Jan
West Bromwich Albion (H) 1 - 1 Niasse - Attendance - 39,061
Wed 31 Jan
Leicester City (H) 2 - 1 Walcott 2 - Attendance - 38,390
Sat 10 Feb
Crystal Palace(H) 3 - 1 Sigurdsson, Niasse, Davies - Attendance - 39,139
Sat 10 Mar
Brighton & Hove Albion (H) 2 - 0 Bong (og), Tosun - Attendance - 39,199
Man City isn't up yet but see here when it is
https://toffeeweb.com/season/17-18/reports/
Edit: so our average is 38,761 and our full capacity is 39,572 so only 811 empty seats on average (a bulk of them probably obstructed views). Seems ok to me lad