Average visitors by month to Venice?

which months are you trying to choose between?

Some of us go there regularly and may be able to advise.

Go to table 5 on page 34.
That shows the presence stats re: the hotels/apartment stays (visitors who stay overnight)

https://www.comune.venezia.it/sites/comune.venezia.it/files/immagini/Turismo/ANNUARIO%202016%20Ita.pdf

But there are a ton more publications that show more statistical data on all possible sauces. Just tell us what you are looking for specifically.

If you are looking to find out whether or not it will be crowded on a certain date, check the crowd estimator on the city website below. In the example below I entered 3/3/2019 and you can tell that Sunday March 3, 2019 willl be very crowded since it’s the last weekend before Fat Tuesday and Carnival attracts lots of folks to Venice.
https://www.comune.venezia.it/it/enjoy-respect-venezia-calendario?dataMiniCalendar=03-03-2019

You probably also want to check cruise ship schedules by date since they dump a lot of people into the city:

http://www.crew-center.com/venice-italy-cruise-ships-schedule-january-june-2019

Thanks for these fantastic links!