Your first step is going to be contacting the reluctant former band kids. You can use COVID as an excuse - "Hey, you couldn't be in band last year due to the pandemic - come on back! We need you!" Use your rosters from the junior highs from last year and in years past.
Your returning kids will NOT be up to scratch for a year or so - but get them in the door. You need second part players too.
Make it about the SOCIAL aspect next year. We went into great depth in SEL this summer - now USE that. Your kids have in many cases been disconnected from EVERYTHING high school now for 18 months. They don't feel like they belong - or they won't have been on campus since spring break of last year. Get them in. Love on 'em. Make them feel good about being in your classroom.
Build on that. Your admin is asking for numbers, NOT a SMC champion. Get kids in the door, go super-simple on your marching show next year, and make it all about having fun.
Use your CURRENT kids to recruit. Tell them all to find one kid that was in band with them in MS that isn't anymore - and bring them to you. Tell them to come to a practice, rehearsal, game, or anything, but see what band is all about. Get them back in, bit by bit.
Do lots of social activities. Make it fun to be in band. Make it the place to be.