Graywolf, I'd be interested to know the benefit to you of having five infirmaries. Do you heal and resurrect alot of troops? I am pondering whether to add another infirmary and I just don't know. I have more silver than I can process.

I only have one now. I don't ressurrect many and with the boosts I can expand it when I'm fighting to heal more. I torched my others and built Manors instead. The silver production is a no big deal to me but the extra boost to training all those mManors make is very welcome.

As with other aspects of the game the number you "need" will depend on how you play. 

I don't know how long you've been playing but in the later stages of the game its almost impossible to have more Silver than you can process. The Aesir will take MOUNTAINS of it by themselves. Manors also give your troop training a boost which is no small thing.

As far as Infirmaries go..... imo and I'm no expert but have played for more than a year (lvl 34 Palace/lvl 70 hero/all Shamans lvl 60) , as far as I'm concerned ressurrecting troops is a waste of Gold. As I mentioned in my earlier post the troop training buffs from Manors are too good to discount irregardless of your Silver situation.

I imagine the OP's question relates to quests but if it concerns general play then received wisdom used to be that resurrecting troops is more costly than training them.  That was based on trivially little actual research - rss cost and availability was ignored for instance.   What is more significant is the change that has taken place whereby 95% discounts now occasionally appear - there is one running as I post.  On the other hand other changes have tended to make training quicker and easier.

One possible answer may be that a bunch of infirmaries will find good usage earlier on in a town's development (assuming the player likes to find opportunities to fight) but will decline in usefullness as knowledge,hero and shaman development and crafting equipment drops the rss cost of training.  So have six or seven while growing but upon reaching, say, 10 billion size expect to convert all but one to manors.

One other factor may be the success or otherwise of your kingdom and clan.  If you regularly win CvCs, KvK, KvKr and Fury you will benefit from bonuses to warrior training time.  That is another small factor which tips the balance towards training.

Firstly, you dont need more than 1 infirmary if resurrecting troops is what you wanna do - Which you shouldnt, unless it is for T1, so you can kill them again and manipulate your trained/loss ratio for traps for example. Or sure, the daily, weekly, monthly quests. 

As for how many, i find 2 infirmaries to be the sweet spot. That lets me HEAL nearly 4m troops each time  with infirmary expander boost, so thats 40% of a 10m march when doing attacks.. which is more or less the maximum percentages you can get from revive warriors boost.. Healing is cheaper than retraining the same number. 

The quest calls for 4. Build your 4 earn your rewards then demolish to the original 1. If the need calls for it later build another. I have found no real need for more than the single Infirmary.

I have 11 infirmary 

with current healing gojng as high as 80%

and huge amount of troops in msrches healing is a very cheap way to replace troops 

1. Healing is very cheap 24/7, no reason to wait for system bonuses

2. You can heal in battle, which turns 100million troops in to 200m troops equivalent if you heal often.

3. RSS cost to heal is much cheaper 

4. allows you to carry lower amounts of troops, hence lower influence but have the same effective power as 2x the troops

example I burned thru 200 million troops the fury but scored over 13 trillion points.

healing  is a huge benefit.