NikolaiD

state = {
  phones: [
    { id: 1, title: '', number: '' },
  ],
}

onChange = ({ target: { value, name, dataset: { index } } }) => {
  this.setState(({ phones }) => ({
    phones: phones.map((n, i) => +index === i
      ? { ...n, [name]: value }
      : n
    ),
  }));
}

addPhone = () => {
  this.setState(({ phones }) => ({
    phones: [
      ...phones,
      {
        id: 1 + Math.max(...phones.map(n => n.id)),
        title: '',
        number: '',
      },
    ],
  }));
}

delPhone = ({ target: { dataset: { index } } }) => {
  this.setState(({ phones }) => ({
    phones: phones.filter((n, i) => i !== +index),
  }));
}

render() {
  return (
    <div>
      {this.state.phones.map((n, i) => (
        <div key={n.id}>
          <input name="title" data-index={i} value={n.title} onChange={this.onChange} />
          <input name="number" data-index={i} value={n.number} onChange={this.onChange} />
          {i ? <button data-index={i} onClick={this.delPhone}>x</button> : null}
        </div>
      ))}
      <button onClick={this.addPhone}>Add</button>
    </div>
  );
}