Can a spellcaster switch between multiple foci?

Question

Background

In a recent session, my cleric was put under the control of a Suggestion spell and was directed to discard his shield, which bears a holy symbol. The enemy knew that clerics use holy symbols as a focus, and attempted to limit my ability to cast spells by removing the symbol.

Due to my background as an Acolyte, I was given a second holy symbol (an amulet) which I wear around my neck at all times. Once the Suggestion spell was broken, I argued that I could still cast spells without providing trivial material components by using the amulet instead of my shield as a focus. After the combat was over, I picked up my shield and began using it as the primary focus again.

Question

Does anything in the rules prevent spellcasters from obtaining multiple spellcasting foci and switching which to use at will?

Answer 1

The rules prohibit nothing - only the DM can prohibit things (that's in the rules PHB p.5).

That said, the relevant part of the rules says (PHB p.151):

A cleric or paladin can use a holy symbol as a spellcasting focus, as described in chapter 10. To use the symbol in this way, the caster must hold it in hand, wear it visibly, or bear it on a shield.

You can use "a holy symbol"; in normal English "a" refers to a general instance i.e. there are more than one of these things and you are referring to any of them right now, this contrasts with "the" where the item is unique.

If you have more than one holy symbol, you can use any of them (but only one at a time - not that that matters). If you find one lying on the ground you can use it. As written, even if it doesn't dipict your god (this is where your DM may prohibit something).

If you have the foresight to carry more than one holy symbol - good for you. If your DM wants to prohibit this then they:

1. can,
2. are being petulent and petty. And, as the DM, they are entitled to be so - you of course are entitled to be petulent and petty in return or rise above and feel the warm inner glow of those of us who inhabit the moral high ground.

If they still need convincing: whould you be prohibited if you had bought a spell component pouch as well as your holy symbol?

Answer 2

There is nothing in the rules that can prevent you from having or using multiple spell foci.

Spellcasting Focus
You can use a holy symbol (see Chapter 5, "Equipment") as a spellcasting focus for your cleric spells.

Later, in the Equipment section, we see possible options for Holy Symbols:

Holy symbol
Amulet      5   gp        1 lb.
Emblem      5   gp        —
Reliquary   5   gp        2 lb.

The description for Holy Symbols in the Equipment section also explicitly allows for the emblem to appear on a shield.

Your character could have a holy reliquary, an amulet, a shield with an emblem, and so on, without any restriction on how many. Since they are not magic items and they don't require attunement there is no restriction.