I prefer to do one at a time, but sometimes I find I'm missing something and have to wait for it, so start another. I have two sitting waiting and one actively in progress at the moment. My next build tends to be decided by group builds - the next two being a 1:72 Meikraft SPAD XIII in Frank Luke's colours, and either a Turkish Fokker E.III (Eduard) or a Bristol M.1C (Pegasus) also in 1:72.
Since I have a house move coming up next week and will finally be moving into my own place instead of renting, these will wait, and after that I'm tempted to get back to the Airfix 1:24 Stuka that I haven't touched in 30 something years, or the 1:12 "Blower" Bentley sitting in my stash for just as long!
The most likely follow ups are a Toko Pfalz XII and Sopwith Salamander, both with PART PE, and the Airfix H-P 0/400 but that may very easily change - there's a tempting Omega Farman Shorthorn in the stash too.....then again, what will the group builds be?
Decisions, decisions....
Ian