Interesting question. My intuition is that it is not grammatical, and may be the result of contamination (the technical term) from other phrases frequently found in news reports such as:
- n people are thought to have died...
- n people are believed to have died...
which both involve passive catenative verbs + perfect infinitivals. (These terms are from the Cambridge Grammar of the English Language (Huddlestone & Pullum); Class 3Aii catenatives include allege, assume, deem, estimate, suspect... ---all acceptable in the above construction.)
However, a cursory Google indicates that number to have died is widespread on leading news outlets such as the BBC and Reuters.
- gauging the number of deaths
- gauging the number of those who have died
- gauging the number of fatalities
- gauging the number of the fallen ;-)