It is said that with Sclerotherapy there is a "High Risk" of having a Stroke. What are the chances for this risk?

Late thromboembolization, years after sclerotherapy is not been described. Once the vein is thrombosed, it scleroses (or scars) and is not likely to embolize months or years later. 

Sclerotherapy is a very safe procedure, although, like any other procedure, that are associated risks.  The risks are diminished by having this done by someone experienced in doing sclerotherapy.  Of all the potential things that can occur from sclerotherapy the risks of stroke and DVT are extremely low.  Stroke or a TIA can occur if foam is used and the patient has a patent foramen ovale. I have never seen a stoke in using foam over 15 years but I have seen patients with a patent foramen.  Similarly with DVT, it can occur but is rare (less than 1% and probably even lower).  Again, I have not seen this occur over years of doing sclerotherapy.Sclerotherapy will not small arteries unless injected into an artery or if there is a connection between a vein and an artery-again very rare.