Although you cannot give a written in stone answer to that question, I was surprised to find an answer online.
The numbers are from 2010, so I guess we would need fewer lenses today, but take a look, and then do the calculations:
If you spend one hour per day, creating four lenses per day, four days per week, how soon could you be making $1,000 per month?
From 0 to $1,000 in 36 Weeks
From those numbers she has calculated an average monthly income per lens of $2.00. The Median is $1.56, so she figures that if we say $1.75 per lens would be reasonable.
It's then a simple calculation to figure out that we need to have 571 lenses to earn $1,000 per month.
Let's say that we create four Squick lenses per day (that's roughly one hour per day), four days a week (remember: the fifth day is meant for editing and improving existing lenses), then we can create 571 lenses in 36 weeks.
Does that sound like a lot of time?
But I discovered something interesting on that Squidoo lens about earning.
The Low Income Lensmasters Gave Up
Take a look at the income Caroline South quotes. Let's start at the bottom.
Almost all of the lensmasters who made less than $1 per lens have created very few lenses since.
Whereas those who made a lot more than $1, have created huge amounts of lenses since.
Greekgeek is quoted for earning $300 per month, but I read somewhere that she makes several thousands now. So it's definitely worth continuing to make lenses.
And I noticed something else: Those who made more than $1 per lens, also updated their lenses more frequently.
Although you cannot give a fixed number of lenses as an answer to the question: how many lenses do I need to make $1,000 per month, there are some points you need to take into consideration, if you want to make money on Squidoo, and according to Caroline South those are:
There are no startup fees
You need to give it at least 6 months
It is a great learning experience
Average earnings per lens are about $1-$2/month
You need lots of lenses
You need unique, researched quality lenses
So have fun, while you're creating your next set of lenses.