According to this BBC News article, it's 4 per 1M:

https://www.bbc.com/news/health-55302595

Quote:

"The MHRA said about four people in a million would normally be expected to develop this particular kind of blood clot - though the fact they are so rare makes the usual rate hard to estimate."

That was my first thought as well. However, both vaccines with this issue are viral vector vaccines. Might be coincidence of course, but it gives me pause.