I'm not sure because I don't know what you really want to achieve with your code, but would that help ?
(using multiple constructors )?
But surely typing Card[ ]  a   ;        would mean "a" is an array.
So you can't type then Card a=………

1. Card[] a, b;
   int numcards = 20;
   
   void setup() {
     size(650, 400);
     rectMode(CENTER);
     f = createFont("Arial", 16, true);
     textFont(f);
     a = new Card[numcards];
     for (int i = 0; i < numcards; i++) {
       a[i] = new Card(i*30 + 15, 60, i+1);
     }
   }
   
   void switchcards(int i, int j) {
     a[i] = new Card(i);
     b[i] = new Card(j);
   }
   
   class Card {
   
     Card(int l, int o, int p)
     {
     }
     Card(int numero)
     {
     }
   }
   

I already have multiple constructors that have worked fine, and I first tried just saying :

I can't debug this without the whole code…
but some people here will maybe able to do this
but it would be helpful and useful to see the whole thing…

I'll upload the rest when I can.

The error message might not be accurate, I fear...
But basically, when you write:
Card a = new Card(a[i]);
it supposes your constructor is taking a Card parameter. If that's not the case (it seems to expect an array of something, apparently), it is normal to have this error.
Since we don't see this class, it is hard to tell more.

Card[] a;
int numcards = 20;

void setup(){
  size(650, 400);
  rectMode(CENTER);
  f = createFont("Arial",16,true);
  textFont(f);
  a = new Card[numcards];
  for(int i = 0; i < numcards; i++){
    a[i] = new Card(i*30 + 15, 60, i+1);
  }
}

void draw(){
  for(int i = 0; i < numcards; i++){
    a[i].update();
  }
  selectionSort();
}

void selectionSort(){
  for(int i = 0; i < numcards; i++){
    int m = 0;
    int j;
    for(j = i; j < numcards; j++){
      if(a[j].num > a[m].num){
        m = i;
      }
    }
  switchcards(i, j);
  }
}

void switchcards(int i, int j){
  Card a = new Card(a[i]);
  Card b = new Card(a[j]);
  for(int k = a.y; k <= a.y + 200; k++){
    a.y = k;
    b.y = k;
  }
  int w = a.x;
  int v = b.x;
  boolean usefulvariable = false;
  if(w > v){
    usefulvariable = true;
  }
  for(int k = 0; k <= abs(w-v); k++){
    if(usefulvariable){
      a.x = w - k;
      b.x = v + k;
    }else{
      a.x = w + k;
      b.x = v - k;
    }
  }
  a[i] = b;
  a[j] = a;
}

class Card{
 
  int x;
  int y;
  int num;
 
  Card(int num_){
    num = num_;
  }
 
  Card(int x_, int y_, int num_){
    x = x_;
    y = y_;
    num = num_;
  }
 
  Card(Card c){
    this.x = c.x;
    this.y = c.y;
    this.num = c.num;
  }
   
  void update(){
    fill(255, 180, 60);
    stroke(0);
    rect(x, y, 28, 50);
    fill(0);
    text("" + num, x-10, y);
  }
 
}

The problem is that you have already a Card called 'a' in global scope, and it is an array, so it cant be assigned to a Card in line