There are generation 1 photosynths, which can still be created via the original Photosynth desktop application for Windows, available from http://photosynth.net/create.aspx
Original photosynths still allow you the freedom to put as many photos as you can fit into a single photosynth.
Generation 2 photosynths, created by the Technical Preview, (the Spin, Walk, Wall, and Pan synth types) have all been simplified to a single line or loop of photos and can only contain a maximum of 200 photos in each synth.
This was primarily to simplify navigation of synths, especially on touch screens.
These limits in Photosynth 2 take away a lot of the freedom that we had in Photosynth 1 to link multiple spins, walks, pans, and walls (and whatever else you could come up with) into one synth and so the Photosynth team has been thinking a lot again recently about linking synths together online but have not yet shipped a proper synth linking feature.