All numbers are a guess. Mexico recently estimated that 23 million of their citizens were here. Take into account that roughly 3+ million illegal aliens cross the southern border yearly, and the BP catches roughly 900k.
Estimates of 30+ million being here are probably accurate.
Thats not the bad news though. All "amnesty" (guest worker) bills allow the illegal aliens to bring their families here, as many are here without their families and send money back to Mexico to take care of them. Amnesty will cost this country a minimum of 75 million new citizens, probably more like 100-120 million new citizens. Consider that roughly half will be eligible to vote.
What this means is that with amnesty your vote becomes worthless. The last Presidential election had roughly 122 million Americans voting. 40-60 million new illegal aliens turned citizen voters means your vote is less then worthless as socialism will be voted in.