ValueError: Number of rows must be a positive integer, not 3.0

I have this code:

import numpy as np
from scipy import stats
from matplotlib import pyplot as plt

if __name__ == "__main__":
    # Create a list of the number of coin tosses ("Bernoulli trials")
    number_of_trials = [0, 2, 10, 20, 50, 500]
    # Conduct 500 coin tosses and output into a list of 0s and 1s
    # where 0 represents a tail and 1 represents a head
    data = stats.bernoulli.rvs(0.5, size=number_of_trials[-1])
    # Discretise the x-axis into 100 separate plotting points
    x = np.linspace(0, 1, 100)
    # Loops over the number_of_trials list to continually add
    # more coin toss data. For each new set of data, we update
    # our (current) prior belief to be a new posterior. This is
    # carried out using what is known as the Beta-Binomial model.
    # For the time being, we won’t worry about this too much.
    for i, N in enumerate(number_of_trials):
        # Accumulate the total number of heads for this
        # particular Bayesian update
        heads = data[:N].sum()
        # Create an axes subplot for each update
        ax = plt.subplot(len(number_of_trials) / 2, 2, i + 1)
    
        ax.set_title("%s trials, %s heads" % (N, heads))
        # Add labels to both axes and hide labels on y-axis
        plt.xlabel("$P(H)$, Probability of Heads")
        plt.ylabel("Density")
        if i == 0:
            plt.ylim([0.0, 2.0])
            plt.setp(ax.get_yticklabels(), visible=False)

            # Create and plot a Beta distribution to represent the
            # posterior belief in fairness of the coin.
            y = stats.beta.pdf(x, 1 + heads, 1 + N - heads)
            plt.plot(x, y, label="observe %d tosses,\n %d heads" % (N, heads))
            plt.fill_between(x, 0, y, color="#aaaadd", alpha=0.5)
            # Expand plot to cover full width/height and show it
            plt.tight_layout()
            plt.show()

I get the error in this line: ax = plt.subplot(len(number_of_trials) / 2, 2, i + 1) ValueError: Number of rows must be a positive integer, not 3.0

I tried to set int(number_of_trials) without success.

Any ideas?