This question needs more specificity. Are you asking over the entire lifespan of Estes, or this year? Do you mean total number of kits produced or how many unique designs were created?
For motors, now, 99%. Probably less 20, 30, 40, 50 years ago when there were more competitors. But now the only other LPR motors are Q-jets. They only just came out last year, and AFAIK aren’t as readily available in hobby stores like Estes motors are. And I’m sure the number of motors Estes sells dwarfs any other motor manufacturer, LPR or HPR.
I tend to think Estes kit volume is *way* higher than everyone else because for a good number of years they've been the only ones with any kind of retail distribution. I wouldn't be surprised if it's higher than 80%.
I love Estes but I'm also happy that there are several other great LPR kit vendors.
I am curious what is the motivation for the question?