am i allowed to install software onto a VM instance?
You get full admin/root permissions on the Azure VM, depending if it is Windows or Linux. I am not aware of any limitation to install software on an Azure VM.
how many ssh connections am i allowed to have at once?
There is a limit of 10 ssh concurrent connections But this can be changed
More details here: https://learn.microsoft.com/en-us/azure/virtual-machines/linux/faq#how-can-i-access-my-virtual-machine
if i use a windows vm, can i install bitvise ssh server?
It should be possible to install bitvise ssh server on a Windows Server. I am not aware of any related limitations installing any software on a Azure Windows VM.
Hop this helps.
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Regards
Andreas Baumgarten