From React 16.8.0 You can use Hooks using useState to instantiate a State custom in your Functional Component. Like This...

import React, {useState} from 'react';

const AddButon = ({handleAddValue}) => {
  return <button onClick={handleAddValue}>Add</button>
}

const App = (props) =>{

  const [value, setValue] = useState(0);

  const handleAddValue = () => {
    const newValue = value+1;
    setValue(newValue);
  }

  return (
    <div>
      <div>The Value is: {value}</div>
      <AddButon handleAddValue={handleAddValue} />
    </div>);
}

If you want to read more about this new functionality, follow the following link.

https://reactjs.org/docs/hooks-intro.html

There is only the (one) 'setState()' method - not a method per-property (as you've suggested/questioned).

It is a composite in terms of it's parameter, in that you can specify/set more than one item within the same/one call (to the 'setState()' method), so you can set all 20 of your items in one go.

E.g.

  this.setState({ "firstName" : firstNameVal, "lastName" : lastNameVal });

I was starting from where you said you started - from a 'class' based component.

If you are sticking with the switch to a 'function' based component, then it is slightly different, in summary:

import React, { useState } from 'react';

...

// The 'useState' hook (function) returns a getter (variable) & setter (function) for your state value - and takes the initial/default value for it/to set it to, e.g.
const [ firstName, setFirstName ] = useState('');

And you then use the getter var to read it & the setter function to set it:

  setFirstName('Dennis');

(I could be wrong but I believe 'hooks' were added in v16.8 of React.)

A more in-depth description:

https://www.simplilearn.com/tutorials/reactjs-tutorial/reactjs-state#:~:text=1%20A%20state%20can%20be%20modified%20based%20on,merge%20between%20the%20new%20and%20the%20previous%20state

states should be like this

const [count,setCount] = React.useState(0); //setState

function count_checkbox() {
    var applicable = [];
    const cbs = document.querySelectorAll('input[class*="checkbox_value"]');
    cbs.forEach((cb) => {
      if (cb.checked) {
        applicable.push(parseInt(cb.value));
      }
    });
    setCount(applicable.length)  //Update State here
  }

setCount sets the state count.