I suppose the 20 Mb is the size of the RAW file on disk. What matters however is the size of the image expanded. I'm just assuming, that Lightroom uses the image in it's RAW coding (with 14 bits per pixel) and you use a 20Mpixel camera. I further assume that LR will expand this to 16 bits for performance reasons. So all the images in memory will take up 2 bytes per pixel. I would further assume that there will be a second buffer of the same size for the final image. And, if Lr works like Ps, Lr will also need a mask for each picture. Using this logic, you easily come to around 4 Gb just to contain the images.
I have to say, that this is pure speculation and I do not know anything about how the stiching algorithm works.
What I know is that Lr needs to find overlapping areas in those 33 images. The time needed for this will increase exponantially. Lr will burn a lot of CPU time with this.