This is a general question, both about the number and composition of field armies as well as garrison and internal/border forces. How were these forces organised and equipped? What was the command structure like? And how did they compare to other militarise? Did they rely on any specific tactics during their various wars (notably the Persian-Roman war of 602-628).
That is a question with a very big answer.
Fortunately there exists online sources which can provide such information in an in-depth manner. The first of these is the Circle of Iranian Studies:
http://www.cais-soas.com/CAIS/Military/sasanian_army.htm
There is also this extensive list of material by Kaveh Farrokh:
http://kavehfarrokh.com/iranica/militaria/militayr-history-and-armies-of-the-sassanians/
The last of these is a text called Persian Sassanian Armies by David Nicolle. I must forewarn you, David Nicolle is very much biased in favour of Early Islamic/Arab culture being superior to everything else (a major reason why I avoid his osprey titles), so take his word with several kilograms of salt:
https://www.scribd.com/doc/24275564/Persian-Sassanian-Armies
Hope that helps.