Slitherine

In playing I had an armor unit that would not give replacements. I had prestige points and other units replaced correctly including other armor units. In next battle, 3 different armor units would not give replacements. Any thoughts why this happened?

Does this have anything to do with prototypes? If so, that negates the value of prototypes.

Ok, found where prototypes have limited replacements. So what is the best way to handle prototypes? Buy them, then when damaged with no replacements, disband them and buy something else?

Well, it depends a lot on the unit, your playstayle and your difficulty settings.
First of all, not every prototype is useful.

Genereally speaking I would be very careful with prototypes and try to protect them as good as you can. If you lose to many units you have to downgrade them again.
There are some speciel heroes that restore one units each turn, if you do not attack, so even if you run out of replacements the unit can come back to full strength.

Never disband units, you will lose all the experience. Just upgrade them to anything else.

As the game currently set up, you cannot see how many spare parts you have for a unit with limited numbers (prototypes, captured units, everything if playing with limited stock). The easiest way to find out is to open the purchase screen. This will show you how many spares you have, not counting units deployed or in reserve. If you don’t see the type of unit in the purchase screen, this means you have exactly zero spares.