You can physically fit a lot of people in a small space, but there are two main constraints of how dense you can occupy a space:
Most office buildings have available parking of 3 to 4 spaces per 1,000 SF of building area. What this means is that your share of the available parking will be at this ratio. If you have more employees (and cars) than your allotment, you could be taking spaces from other tenants and stressing out the parking lot. As a building approaches 100% occupied, over parking becomes more of a problem for management as existing tenants start to complain. If you become a parking nuisance, the landlord has the ability to put you into default of your lease.
If you put a bunch of people and computers in a small space (in Texas), the standard air conditioning can’t keep the space cool. The landlord isn’t going to fix the problem if you are over your allotted density.