Irregular subplots in matplotlib python3

Question

I plotted 3x4 subplots and I would like to add one more - to the first row (for instance to the position 1 4. How to do that, please? I tried the comment lines and I obtain an error:

AttributeError: 'numpy.ndarray' object has no attribute 'set_xlim'  

Code:

import numpy as np
import matplotlib.pyplot as plt

fig, ((ax1, ax2, ax3, ax4), (ax5, ax6, ax7, ax8), (ax9, ax10, ax11, ax12)) = plt.subplots(nrows=3, ncols=4, figsize=(12,7))
#fig, ((ax0), (ax1, ax2, ax3, ax4), (ax5, ax6, ax7, ax8), (ax9, ax10, ax11, ax12)) = plt.subplots(nrows=4, ncols=4, figsize=(12,7))

for ax in [ax1, ax2, ax3, ax4, ax5, ax6, ax7, ax8, ax9, ax10, ax11, ax12]: 
#for ax in [ax0, ax1, ax2, ax3, ax4, ax5, ax6, ax7, ax8, ax9, ax10, ax11, ax12]: 
    ax.set_xlim(0,1000) 
    ax.ticklabel_format(useOffset=False)

plt.show()