There are 4 MOD slots at every portal and you can deploy a single portal mod in every of them. To deploy a portal mod, you need to be at action distance from the portal. That portal will also need to be in possession of your faction and have a free mod slot. A max of 2 mod slots may be used by a single agent on a single portal. Each consecutive portal mod has less value when deployed, so the first shield will give more of a stat benefit than the second one.
With the new Portal Mods it’s important to find a good balance and experiment with what combination of Mods work best for you and your needs.