I cannot find any statistics from before 2012, but in the first quarter of that year, 13,000 PA citizens were working in the settlements.
The latest report from the Palestinian Central Bureau of Statistics says:
The number of workers from the West Bank working in Israel and settlements is now 110,300 workers in the first quarter 2015 compared to 105,200 workers in the fourth quarter of 2014...That is a 61% increase since the beginning of 2012, and the numbers since the law was enacted in 2010 are probably more dramatic - possibly as much as a 100% increase.
The number of employees in the Israeli settlements increased from 20,200 workers in the fourth quarter of 2014 to 20,900 workers in the first quarter 2015.
These are the highest numbers of both workers in Israel and in the settlements since the second intifada as far as I can tell.
The reason is easy to see. Israelis pay more than double the wages that Arabs do.
The report says that the average daily wage in the West Bank is 94.2 shekels, versus 196.4 shekels for those working for Israelis. And the ones in Israel average two fewer hours per week of work.
About one in nine West Bank workers is employed by an Israeli.
This means that close to 25% of all wages earned by West Bank Arabs comes from Israeli employers.
The boycott does not seem to be working very well. And if the law would be enforced against working in the settlements, the unemployment rate in the West Bank would rise by about two percentage points, which is significant.
The PA is trying to find Arab countries who can employ Palestinians, but with the wars in Yemen and Syria this is not working out too well for them either.