HI I AM SO SORRY THIS REPLY IS A LITTLE LATE I FORGOT ABOUT RETROSPRING I HOPE YOU STILL SEE IT!!!!!
so the answer for serial code chuusen changes pretty significantly based on venue/popularity/what kind of ticket you’re going for. for example, i got an S ticket for mygo at bandori 12th using just three serial codes, but it took me ten to get an S ticket (or w/e the equivalent ‘best ticket’ is) for their zepp haneda tour stop and eleven to get one for divide/unite. in all three instances i would say i got the bare minimum ‘safe’ amount to secure the seats i wanted.
based on these factors, the answer again will vary. i’m assuming you’re not going for mygo 7th as chuusen had closed even before i got this—if you are interested in stats about how this chuusen went, someone did a questionnaire asking people how many codes they entered/if they won/etc., i can dig it up if you like, but i’m guessing it probably took 20 serial codes to secure S seats (if you weren’t going for S seats i think chances were way higher). so for the only upcoming events i can think of:
avoid note (mygo x togetoge): this is in a venue too small for just one of them, much less both. since serial codes are w BDs for this one it’ll be more expensive so it probably won’t be as high as it would be if it was CD chuusen instead but i would seriously do at least 20 but probably 30 codes, especially if you want a shot at better seats (though we don’t know seating details at this point i don’t think)
mygo x ave mujica: personally i’m probably shooting for 25 codes, but that very well may be overkill—mymuji are popular and rising, but it’s a two day live, the venue is 20k which is the biggest venue either has headlined afaik, and chuusen is before the anime which means less hype so chuusen won’t be as brutal (hopefully); i just refuse to miss out on this chance At All so i’m going hard lol. we also don’t know a lot of the details for this though (incl if serial codes can be used for both days), so this could change too

also! if you’re going to mymuji and would like help w chuusen, please message me (if you feel comfortable ofc)! i am p sure i know a number of people going and want to coordinate chuusen purchasing efforts to maximize potential winnings and make sure everyone gets a ticket who wants without having to spend a totally heinous amount all by oneself. so for anyone reading this interested please reach out!!

(also sorry if this answer sucks i may edit it later i am about to get off my break and just don’t have the time rn to do this properly but i don’t want to forget to respond fhdkdjs)