\documentclass[12pt,a4paper,oneside]{book}
\usepackage{amsfonts, graphicx, verbatim, mathtools,amssymb, amsthm, mathrsfs,amsmath}
\begin{document}
\begin{align}
\diff{S}{t} &= \Lambda -\beta(1-\rho)(1-\xi)SI -(\mu+\rho+\xi)S,\\[1ex]\nonumber
\diff{I}{t} &=\beta(1-\rho)(1-\xi)SI-(\mu+\alpha+\theta+\gamma )I, \\[1ex] \label{1}
\diff{R}{t} &=(\theta+\gamma)I+(\rho+\xi)S-\mu R,\nonumber
\end{align}
\end{document}
How would I put the number in "middle" of these equations? here is is nice as there's 3 equations but what if there was 2,4 or 5 equations?
EDIT: Using Egreg's Comment on the following set of equations:
\documentclass[12pt,a4paper,oneside]{book}
\usepackage{amsfonts, graphicx, verbatim, mathtools,amssymb, amsthm, mathrsfs,amsmath}
\begin{document}
\begin{equation}
\begin{aligned}
d S&=\left[\Lambda(r(t))-\beta(r(t))(1-\rho(r(t)))(1-\xi(r(t)))SI-(\mu(r(t))+\rho(r(t))+\xi(r(t)))S \right] d t\\
&+\sigma_1(r(t))S d B_1+\int_M D_1(r(t),y) S(t-) \tilde{N}(dt,dy),\\
d I&=\left[\beta(r(t))(1-\rho(r(t)))(1-\xi(r(t)))SI-(\mu(r(t))+\alpha(r(t))+\theta(r(t))+\gamma(r(t)))I \right]d t\\
&+\sigma_2(r(t)) I d B_2+\int_M D_2(r(t),y) I(t-) \tilde{N}(dt,dy),\\
d R&=\left[(\theta(r(t))+\gamma(r(t)))I+(\rho(r(t))+\xi(r(t)))S-\mu(r(t))R \right]d t+\sigma_3(r(t)) R d B_3\\
&+ \int_M D_3(r(t),y) R(t-) \tilde{N}(dt,dy).
\end{aligned} \label{7.2}
\end{equation}
\end{equation}
produces one number but it is not centralised, how can we fix this?
\begin{equation}\begin{aligned}<same input>\end{aligned}\end{equation}But remove\nonumberand place\labeloutsidealigned. – egreg Oct 31 '22 at 15:22! LaTeX Error: Environment align undefined.– David Carlisle Oct 31 '22 at 15:26