Bought some dehumidifier  packets from Amazon, how much packets to you put in a ammo can?

The best way to figure that out is to put a hygrometer in the can and open it a few days late to see what the reading is.  You should be able to get the moisture level below 25% in a sealed ammo can.

https://www.amazon.com/Goabroa-Hygrometer-Thermometer-Temperature-Fahrenheit/dp/B08HWG68TG/ref=sr_1_10?dchild=1&keywords=humidity+meter&qid=1614918617&refinements=p_72%3A2661618011&rnid=2661617011&sr=8-10

Bought some dehumidifier  packets from Amazon, how much packets to you put in a ammo can?

Several things to consider to answer that question:  can size, condition of seals, condition of can, contents, and frequency of use.

I use a 40 gram reusable desiccant pack -- one per ammo can (30 and 50 cal sizes).  They run about $5-$8 each, less if you find them on sale.

According to the listing on MidwayUSA, they offer 3 cubic feet of coverage per unit.

https://www.midwayusa.com/product/1022530137?pid=465276

The seals were new when I bought the cans over the last 4-6 years, so humidity from the outside was not a problem.  Just what got trapped if I opened a can.  That's why I had "Range" cans for each caliber and the rest were long term storage (rarely opened).

I've read that cardboard absorbs moisture, so I remove all cardboard boxes and put the ammo in Ziploc bags.

I check the desiccant every 3-6 months.  They needed to be dried out in an oven when the material turns pink (some others turn green when saturated).  I probably reactivate the long term storage packs every 9-12 months or longer.  The range cans need reactivating every 2-3 months when I was going to the range once or twice a month.

Hope that helps.

Just looked your product up on Amazon.  If I found the right one, the description says:

Packet dimension: 2 inch x 2 ¾ inch); 5 gram desiccant packets can cover
approximately 600 cubic inch volume containers; We recommend placing
more packets than recommended for extra protection

A 50cal ammo can is 540 cubic inches (12x7.5x6). 

So, theoretically, one 5 gram pack can be used in one can -- 30 or 50 cal.  The questions then become: how often are you going to open the lid, and how air-tight are the cans?

There's really no way to know how much moisture is trapped in the can after closing.  That's why the silica changes colors.  When you see the color change, time to reactivate them.

Thanks for the tips.