There two electrons in an atomic orbital, so shouldn't atomic orbital be two electron wave function?
No, it should not. The thing is that the actual one-electron wave function is the so-called spin orbital which is a function of not only spatial coordinates of a single electron $(r,\theta,\phi)$ but also of its spin coordinate ($m_s$). Spin orbitals are constructed from spatial orbitals by using the so-called "spin up" and "spin down" functions defined as follows:
\begin{equation}
\alpha(m_{s}) =
\begin{cases}
1, & m_{s} = +1/2 \\
0, & m_{s} = -1/2
\end{cases} \, ,
\quad \quad \quad
\beta(m_{s}) =
\begin{cases}
0, & m_{s} = +1/2 \\
1, & m_{s} = -1/2
\end{cases} \, ,
\end{equation}
and in general chemistry it is assumed that spin orbitals always come in pairs - from each spatial orbital $\chi$ two spin orbitals are constructed:
\begin{equation} \label{eq:rhf_spin_orbitals}
\begin{aligned}
\psi_{\alpha}(r,\theta,\phi,m_s) &= \chi(r,\theta,\phi) \alpha(m_s) \, , \\
\psi_{\beta}(r,\theta,\phi,m_s) &= \chi(r,\theta,\phi) \beta(m_s) \, .
\end{aligned}
\end{equation}
Thus, both electrons that occupy the same spatial orbital (say, atomic one) are described by the wave functions (spin orbitals) that share exact same spatial part and this spatial part (spatial orbital) still is a one-electron function in a sense that it depends on spatial coordinates of a single electron only. Note that this apparent paradox arises because calling spatial orbitals wave functions is strictly speaking incorrect - the spin orbitals are the actual wave functions, not spatial ones.
Distance is from which centre if not from centre of nucleus? Also why
distance from centre is proportional to ΘΦ?
This is tough one without the context. Can share more text with us or reference a book you are apparently reading?
Lastly, I guess, it is impossible to plot the variation of 𝜓 with
𝑟,𝜃,𝜑, because it would require 4 axis (or four-dimension graph).
Am I right?
This is obviously true. Being limited with plane sheets of paper we can only draw two-dimensional graphs.