Hello, welcome to JustAnswer! I am a Veterinarian and will help you right away!
This isn't as easy an answer as it appears.
In dogs the gestation generally is about 2 months, or roughly 63 days.
This is 63 days from the time of ovulation
65 days from the LH surge, which is the hormone surge that releases the egg from the ovary
63 days (+/-) 7 days from the first breeding date
Or when doing vaginal cytology before planned breeding, it is about 57 days from the diestrus
Lots of stuff that you probably weren't even thinking about, and probably isn't helpful.
SO, I will tell you that it's usually 60 days give or take a week before of after from the time the bitch is bred.
Bostons tend to go pretty close to 60 days, but it depends on litter size. When the litter sizes are larger, the pups come closer to 57 days. When the litter size is smaller it will be later, like 63 days.
What you will want to do is get a radiograph of the bitch at about 45 days of gestation. This will tell you if she is pregnant and help get a count on the number of pups. You can see them earlier with ultrasound starting at around 35 days, and a skilled vet can also feel them in the abdomen.
Ultrasound is 99% accurate at diagnosing pregnancy at around 30 days.
Another reason to have an xray is because Bostons often need cesarian sections. It's nice to know how many pups are in there so when she goes into labor, and three pups are out, and you know there are 4 from the xrays, then you will know it is an emergency if the fourth doesn't appear after 2 hours of pushing. So, get that xray.
According to the AKC, Boston Terriers typically have 4 pups with the range being 2-8.
Of course, those numbers are like saying people typically have one baby, with the range being 1 to 10. It just depends on the breeding. Get the xray, that will tell you how many are in there.
As always, breeding these dogs is complicated and there is more to it than just hooking her up with a male. Hopefully you have chosen your male responsibly and with the best possible genetic outcome in mind.
These pages will further assist you :http://www.boston-terriers.com/csection.htmhttp://www.bostonterrierhub.com/boston_terrier_breeding.html