I don't think we have that figure with any kind of statistical precision, but we know there is a very serious risk of dying in custody in China from a combination of the effects of torture. Certainly over the years this has been well documented across all population groups that experience imprisonment and torture in China. These would include Uighurs, Falun Gong practitioners, Tibetans, and so on. The torture is very frequently so severe and the access to medical care afterwards so restricted that there have been significant numbers of individuals who have died in custody because of the torture.
Added to that, there are always concerns about prison conditions and people's health situations. People may have had health problems before they were detained, and those are exacerbated by very poor prison conditions and a lack of access to medical care. Or they may simply develop health conditions because of the poor prison conditions. Certainly sometimes those concerns can become grave enough that people die as a result.
Imprisonment in China is unjust for many reasons, but it can actually have life or death consequences.