Do you know if historical statistics from area51 are available? For instance, how many users we had yesterday and how many the previous day?
Yes... sort of.
Now that beta sites are part of the Data Explorer, it's possible to extract nearly all the Area 51 statistics for nearly time period you care to look at. Here's questions/day for Open Data:
You'll have to construct your own queries for the other numbers. Remember that since the data dumps are weekly, the Stack Exchange Data Explorer (SEDE) results will be missing up to 7 days of data.
The data that isn't readily available is traffic. For those interested, Quantcast does publish visitor data, but I believe it's measured differently than the numbers used for Area 51.