The classic Milnor-Thom upper bound on the sum of the Betti numbers of real algebraic sets (for a nice exposition and references, see e.g. N.R. Wallach, On a Theorem of Milnor and Thom, in S. Gindikin (ed.), Topics in Geometry: In Memory of Joseph D'Atri, Progress in Nonlinear Differential Equations and their Applications 20 (Birkhäuser, 1996), pp. 331-348) allows one to give an upper bound on the number of connected components of the complement $$ U_P=\{\xi\in\mathbb{R}^n\ |\ P(\xi)\neq 0\} $$ of the zero set of a real polynomial function $P:\mathbb{R}^n\rightarrow\mathbb{R}$, since $U$ is homeomorphic to the algebraic subset $\tilde{U}_P=\{(\xi,\lambda)\in\mathbb{R}^{n+1}\ |\ \lambda P(\xi)=1\}$ of $\mathbb{R}^{n+1}$ through the map $$ U_P\ni\xi\mapsto(\xi,P(\xi)^{-1})\in\tilde{U}_P\ . $$ The aforementioned bound tells us that if the degree of $P$ is $r$, then the sum of the Betti numbers of $U_P$ (and, therefore, its number of connected components) is bounded above by $(r+1)(2r+1)^n$.

Question: is there an upper bound on the number of convex connected components of $U_P$ which is sharper than the Milnor-Thom bound but also only depends on $n$ and $r$?

My intuition is that the Milnor-Thom bound is too crude to this end, even in the exceptional case when all connected components of $U_P$ are convex - take, for instance, $P(\xi)=\prod^n_{j=1}\xi_j$, in which case $r=n$ but the connected components of $U_P$ are precisely the $2^n$ orthants $$ \mathbb{R}^n_I\doteq\{\xi\in\mathbb{R}^n\ |\ \xi_j>0\text{ if }j\in I\ ,\,\xi_j<0\text{ if }j\not\in I\}\ ,\quad I\subset\{1,\ldots,n\} $$ of $\mathbb{R}^n$, which are clearly convex.

My motivation for the above question is related to Lars Gårding's theory of hyperbolic polynomials (see e.g. L. Gårding, An Inequality for Hyperbolic Polynomials, J. Math. Mech. 8 (1959) 957-965). Recall that an homogeneous polynomial $P:\mathbb{R}^n\rightarrow\mathbb{R}$ is said to be hyperbolic with respect to $0\neq\tau\in\mathbb{R}^n$ if $P(\tau)\neq 0$ and the roots of the one-variable polynomial $P_{\xi,\tau}(\lambda)\doteq P(\xi-\lambda\tau)$ are all real. The hyperbolicity cone $C(P,\tau)$ to which $\tau$ belongs is the connected component of $U_P$ to which $\tau$ belongs. Gårding has shown that $$ C(P,\tau)=\{\xi\in\mathbb{R}^n\ |\ \text{all roots of }P_{\xi,\tau}\text{ are positive}\} $$ and, as a consequence, that $C(P,\tau)$ is an open convex cone. Roughly speaking, dual cones of hyperbolicity cones provide upper bounds to the propagation speed of the support of solutions of hyperbolic partial differential equations with principal symbol $i^rP$.

Due to the homogeneity of $P$, hyperbolicity cones always come in opposite pairs - more precisely, if $P$ is hyperbolic with respect to $\tau$, then it also is with respect to $-\tau$, and $C(P,-\tau)=-C(P,\tau)$. Since $P$ in the above example is hyperbolic with respect to any $\tau$ in $U_P$, we conclude that the number of hyperbolicity cones of an hyperbolic homogeneous polynomial $P:\mathbb{R}^n\rightarrow\mathbb{R}$ is an even number bounded above by the number of convex connected components of $U_P$, and such an upper bound is sharp. Therefore, a positive answer to the above question would provide an upper bound to the number of hyperbolicity cones of $P$ which is sharper than the Milnor-Thom bound.