For a startup company I thought of going with Ubuntu operating system for 200 desktops. To reduce cost to the company I don't want to rely on Microsoft. Can I go ahead and install Ubuntu 18.04.3 LTS for all my employees and run my office? I don't want to encounter any legal issues.
Yes it is free as stated many times here:
In a 200 person company at $100K per head (salary + benefits + cost of desk space) it's a 20 million per year cost just for that plus inventory, warehousing, offices, consultants and contractors. The $50 cost x 200 machines for Windows is the least of your concerns.
Your computer systems will cost millions for accounting, operations and mission critical applications.
Your senior executives will cost $250K + and will come with their own preferences for JD Edwards running on AS/400, or SX.e running on Windows/Linux or Navision on Windows, or SAP on a variety of platforms.
Chances are 100 to 1 the people you hire that know spreadsheets (a big part of business) will know Microsoft's Excel and not Ubuntu's offerings.
So the cost of $50 / seat Windows 10 is really a moot point IMHO.
The $10,000 for the Windows OS on a volume license is just the start of your Windows related costs. For volume licensing, you will need Microsoft file servers, license servers, and an abundant number of other servers, all of which will need licenses from Microsoft. You will also spend a significant number of staff-hours per week maintaining that software, which is significantly more vulnerable to attacks than a proper Ubuntu-based network.
Also, the cost for 'inventory, warehousing, offices, consultants, and contractors' previously cited may not pertain to your business model. By all means, do budget for those if you must have them, but I won't speculate on what you will or won't need in whatever your line of work will be. I will, however, expect the cost of operating and maintaining an Ubuntu-based network will be even greater a savings than the savings from a free OS and free applications.
I foresee no legal issues, but an important thing to remember is getting support. Canonical offers enterprise support but you'd need to pay for it. Of course you'd need to pay for this for Windows as well. My point is that there's more to the equation than merely the license to run the software.
Another thing to consider is that Ubuntu is unfamiliar to many people. In a start up environment I guess this will be less of an issue (I assume people are more tech savvy and willing to learn) but it's something to consider nonetheless.