First of all, I think the challenge is that there is no kind of internationally accepted list regarding what is a tax haven. There are lists that various international organizations have established of non-co-operative jurisdictions, those that don't meet the standards of transparency and so on. Those tend to be fairly short lists.
There are other perceptions of low-tax jurisdictions, which some people might consider tax havens or which might be so called in common parlance. There's no common terminology accepted throughout the world as to what is a tax haven.
The Canada Revenue Agency has done estimates of the tax gap, and I can undertake to ask my colleagues at the Canada Revenue Agency to provide those. Those are not necessarily responsive to your question entirely, because there are tax gaps on the corporate side and so on that include more than just what one might consider planning and tax havens.