You can go back a few pages on this thread and read some trip reports from the last few parties of the 2019 season. There are a few things to keep in mind though....
1. "crowded" is a definition dependent on an individual's opinion. I saw LOTS of complaints during the whole party season last year about how "crowded" the parties were. Even on the nights that weren't sold out. I fully expect the crowds to be larger the closer you get to Halloween, but I'm really using the "sold out" status to define "crowd".
2. We never know ahead of time (with the exception of Halloween itself and probably the first one with all the vloggers/bloggers) if a party will sell out. Last year there were 13 sold out parties (the most recorded so far on here) out of 36 total parties (also the most recorded so far on here).
3. Will Disney be actively promoting the party like they did in 2019 with kiosks in the monorail resorts? We don't know how many tickets they actually sell to know how many people are in a "sold out" party.
4. Will Disney offer the Party Pass this year? The deal was incredible last year and people took advantage of it... but we don't know how many party passes they sold or when those people were in the parties. We also don't know if Disney factored those people into the equation when they listed a party as "sold out". If they offer the party pass again, the same way without restrictions (except for Halloween itself) there's no way of knowing ahead of time how crowded a party could be.
If you're interested in a party, I would say go for it! It really is a lot of fun! Just have a good idea of what you want to accomplish and be flexible. I've been to parties in August, September and October and I definitely think the parties on October are more fun. People really seem to get into it more and the costumes become more elaborate.