Can not open more volumeFiltering.exe

I just open volumeFiltering.exe(or SobelFilter.exe or volumeRender.exe)again and again and do not close them.But I find that I can not open the 29th one,so there are only 28 volumeFiltering.exe running at the same time.About 3000 MB memory of the video card has been used,so the memory size of the video card seems not the reason.
I also try it with Quadro K4000,and the number is not fixed ,it is near 45,and it is limited by the memory of video card since K4000 only has 3072MB memory.
So what is the reason that I cant not open the 29th one or more?

My Computer:CPU:E5 2620v2 * 2, Memory: RECC Memory 32GB,Video card GPU GTX980 *2(SLI closed).OS:Windows 7(64bit),Display Drivers Version:343.98 CUDA Version:CUDA6.5 For GTX 9XX

I find that I can open SobelFiltering.exe many times till all the memory of the graphics card is used on GTX 760 or Quadro K4000.But the problem is still there when I change to use GTX 980 or GTX 750Ti.
I guess it is because of the architecture since GTX 760 and Quadro K4000 have Maxwell architecture,GTX 980 and GTX 750Ti have Kepler architecture.But I am not sure.
So how can I soleve this problem?