> QUESTION: How many machines does my Maxwell Render Standard license support?

Each Maxwell Render Standard license is valid per a single machine with unlimited number of cores.

> QUESTION: Can a license be transferred from Win to Mac to Linux? Any special procedure for that or can I simply download Maxwell for Mac/Linux and use my existing license.dat file?

Yes, with a single license, you can work on Windows, Mac and Linux. You can download Maxwell for each operating system and use the same license file.