AT&T Cell Booster, Can I set up more than one of them in my office building?

In theory yes. With the MicroCell, you could use two MicroCell's as long as they were at least 100' (preferably longer) apart. Given the coverage with the Cell Booster, I would use 100' as the minimum and try from there. Keep in mind if you don't get adequate coverage, it could be due to location, building materials etc so a second Cell Booster in the same area might not cover like you want. Additionally, you can not transfer active calls from one Cell Booster to another, and there is no way to force your phones to pick one Cell Booster over another. Your phone may "lock onto" the last Cell Booster it was connected to initially so if it was on one side of the office and you want to initiate a call on the other side, there may be issues.

You can have up to three Cell Boosters on one account.

What about WiFi-C (WiFi Calling), which is actually preferred over the Cell Booster.

I have two on one campus but they are in different buildings about 400' apart. As @OttoPylot says Wi-fi-C is probably better if that's a solution for you and no boosters or devices needed just turn it on in your phones settings. Or you could look into the Cell Booster Pro which has 15k sqft coverage as opposed to the regular one's 3k sqft coverage.