You can place as many on the field as you wish. That is to say, if you somehow had 10 Spell Cards in hand, you are allowed to activate them all - you'd activate one, let it resolve, and it would go to the graveyard after.
Keep in mind though you have only 5 regular S/T slots, and a Field Spell Card slot. If you activate any continuous Spells, or Set a spell, then these are taking a slot up. If you fill all your slots like this, you cannot activate any other cards until a slot is freed up.
You can't ever just discard cards for no reason, you can only do so if instructed to by a card effect or a game rule, such as the one that lets you replace one Field Spell Card with another.
there is no limit
No, there isn't. Unless you count a Field Spell Card, which is a spell card but has its own zone, so technically you can have 6 spells active.
Depends on the field spell. Inherently, Armityle has no effect that says he is immune to the effect of field spell cards.
You may replace your active Field Spell card with a new one, either Setting it or Activating it. Your old card is destroyed and goes to the graveyard. Replacing an active Geartown by Setting a second one is a focal combo in Geartown decks. If your opponent has a Field Spell card active, this is destroyed when you resolve a Field Spell card. However their active one is not destroyed if you just Set one, and neither is their Set one destroyed if you activate or Set one too.
You are only allowed 11 cards as a maximum on a field, you have five monster card slots, five spell/trap slots, and a Field Spell Card slot.
You can't ever just discard cards for no reason, you can only do so if instructed to by a card effect or a game rule, such as the one that lets you replace one Field Spell Card with another.
there is no limit
Yes, both Mystical Space Typhoon and De-Spell can destroy a Field Spell card in Yu-Gi-Oh. When you activate these cards and target a Field Spell, you can destroy that Field Spell card.
No, there isn't. Unless you count a Field Spell Card, which is a spell card but has its own zone, so technically you can have 6 spells active.
The Graveyard is part of the Playing Field. When a card states "...on the field...", it is referring to cards that are placed in the Monster Card Zone, the Field Spell Card Zone, and the Spell & Trap Card Zone.
Depends on the field spell. Inherently, Armityle has no effect that says he is immune to the effect of field spell cards.
You may replace your active Field Spell card with a new one, either Setting it or Activating it. Your old card is destroyed and goes to the graveyard. Replacing an active Geartown by Setting a second one is a focal combo in Geartown decks. If your opponent has a Field Spell card active, this is destroyed when you resolve a Field Spell card. However their active one is not destroyed if you just Set one, and neither is their Set one destroyed if you activate or Set one too.
You can, only if something specifically says you can. Tribute for most Ritual Spell Cards can come from hand. But if the card doesn't specify, the tribute must come from your side of the field.
A Field Spell Card is destroyed when another Field Spell Card is activated or via an effect, such as Dust Tornado. If you activate a Field Spell Card while there is another already active on the field, your Field Spell Card will destroy and replace the existing one.
yes
Almost. It's text clearly states that it "negate(s) the effect of all spell cards on the field". So as long as it's a spell card active on the field, it is no good. Some cards, however, activate when they are sent to the graveyard, but those are rare. but if u use the effect of destiny hero diamond dude then silent sworsman cant negate it