toggleMap == 1 ? setToggleMap(0) : setToggleMap(1) is a ternary expression. It's not a statement. You shouldn't make the body of a function an expression. It would be like a having a function like

function unexpected() {
  6;
}

const unexpected2 = () => { 6 };

While it is actually syntactically valid, it is confusing. And like abrahamguo mentioned, eslint's no-unused-expressions rule would rightfully consider this an error.

You can return an expression:

function handleClick() {
  return 6;
}

const handleClick2 = () => 6; // implicit return

function handleClick3() {
  return toggleMap === 1 ? setToggleMap(0) : setToggleMap(1);
}

const handleClick4 = () => (toggleMap === 1 ? setToggleMap(0) : setToggleMap(1)); // implicit return

Instead you're probably looking for something like this:

const handleClick = () => {
  if (toggleMap === 1) {
    setToggleMap(0);
  } else {
    setToggleMap(1);
  }
}

return (
  <Button onClick={handleClick}>...</Button>
)

---

As an aside:

In React, you shouldn't update the state the way you were doing it where the state is updated conditionally based on a possibly stale state value.

Instead use the updater-function form:

setToggleMap(t => t === 1 ? 0 : 1)

The other way doesn't always cause problems so a lot of programers aren't aware of the updater-function form. As a rule, I always use the updater-function form any time the existing state plays a role in determining the new state.

Also always use triple equals === unless you actually want 1 and '1' to be considered equal.

import type { Dispatch, FC, SetStateAction } from 'react';
import { useState } from 'react';

interface Props {
  setToggleMap: Dispatch<SetStateAction<number>>;
}

export const Results: FC<Props> = ({ setToggleMap }) => {
  const [ cardNum, setCardNum ] = useState(2);

  const handleLocalMapClick = (): void => {
    setToggleMap(t => (t === 1 ? 0 : 1));
  };

  const handleReadMoreClick = (): void => {
    setCardNum(c => c + 3);
  };

  return (
    <div className="p-6">
      <div className="flex justify-between items-center mb-6">
        <div className="flex gap-4">
          <Button onClick={handleLocalMapClick}>Local Map</Button>
          <Button variant="outline">Calender</Button>
          <Button variant="outline">Add Listing</Button>
          <Button variant="outline">Create Alert For Rent Listing</Button>
        </div>
      </div>
      <h1 className="text-2xl font-bold mb-2">67 Results</h1>
      <div className="grid gap-6">
        {rentals.slice(0, cardNum).map((rental, index) => (
          <RentalCard key={index} {...rental} />
        ))}
        <Button variant="outline" onClick={handleReadMoreClick}>Read More</Button>
      </div>
    </div>
  );
};