Recall that the cyclotomic polynomial of order $n$ is $$\Phi_n(X)=\prod_{gcd(k,n)=1}(X-e^{2ik\pi/n}).$$ Its degree is $\phi(n)$ (Euler's indicator). Inversion of the Moebius formula $$X^n-1=\prod_{d|n}\Phi_d(X)^{\mu(n/d)}$$ implies that $\Phi_n\in{\mathbb Z}[X]$. The height of $\Phi_N$ is the maximal modulus of its coefficients. Thus the height of $$\Phi_p(X)=X^{p-1}+\cdots+X+1$$ ($p$ odd prime) is $1$. A cyclotomic polynomial is flat if its height is $1$. We know that the height of $\Phi_{2n}$ equals that of of $\Phi_n$, and also that if a prime $p$ divides $n$, then the height of $\Phi_{pn}$ equals that of $\Phi_n$. Therefore it is enough to analyse the case where $n=p_1\cdots p_\ell$ is the product of distinct odd primes. When $\ell=2$ it is known that $\Phi_{p_1p_2}$ is flat. It is known that there are infinitely many flat $\Phi_{p_1p_2p_3}$, but $\Phi_{105}$ ($105=3\cdot 5\cdot 7$) is not flat (its height equals $2$).
Question. What is known about the growth of the height of $\Phi_n$ ? Is there a bound of the form $C_\ell$ (thus extending $C_1=C_2=1$) ?
