Hi there,
Pretty sure it’s four, but may be addition consultants. and some jr. eggineers 
and Techs.
No idea on turn over though.
Pretty sure based on my experience here, NO title’d Software Engineers though. Mainy Hardware or part time software group or responsibilities by the whole team. IDK , there has never been an official introduction or “this is the TEAM at Seeed”
They and WE would definitely benefit if they had a designated Software Engineering Manager and a couple SE tech’s to “clean up, catch up and make current” the Software Ball of Yarn they have ended up with due to so many products that relay on a solid software foundation. Hence they are lacking obvious by the questions posted Daily on the forum. 80-90% are software related.
They make and procure Great hardware… the other NOT so Much. one example is the Slick demo for the RGB bike Light, Still no official Software or wiki… because it feels like to me Software is not a priority. (tisk, Tisk) Software engineers are EXSPENSIVE and good ones are elusive.
They are getting a taste of that I feel, with the whole AI debacle , so many moving parts to it and NO clear Guide with repeatable and customisable solutions. But the watcher is almost out…LOL
Even the fruit people have a dedicated SE dude that generates there LIB’s and releases test code that works but only and craftly with their own custom hardware, so it’s of little use to General Public only DaFruit hardware fanboys. They are protectionist that way ie. Are not Open Source and build a wall for only their hardware use. just use Bootloader as example.
Nothing new here, (soapbox off) moving ON.
GL 
PJ 
(disclaimer)
The views expressed are my own. All opinions are my own. The opinions expressed here belong solely to me and do not reflect the views of any company official or it’s actions…