Is there an explicit bound on the number of tetrahedra needed to triangulate a hyperbolic 3manifold of volume $V$? Or more generally a hyperbolic $n$manifold of volume $V$?

A couple of things are true: 1. If you have any Riemannian manifold of bounded infinitesimal geometry (curvature pinched above and below), its thick part, where the injectivity radius $> \epsilon$, can be triangulated with a number of simplices bounded by a constant times volume, where the constant depends on the curvature bounds and the dimension. I don't personally know the constant even for hyperbolic 3manifolds, but I think there are people who can produce explicit bounds. This is basically a consequence of the compactness of the set of manifolds of bounded infinitesimal geometry and injectivity radius bounded below, together with the fact that all smooth manifolds admit a smooth triangulation, and that any smooth triangulation of a closed subset can be extended.
The answers are the same whether you're asking for a geodesic triangulation of a hyperbolic manifold, or any smooth triangulation. 

