lwc.html

<template for:each="{forecastRevenues}" for:item="period">
  <tr key="{period.index}">
    <td
      data-label="{period}"
      data-id="{period.Id}"
      data-field="Forcast_Period_Start__c"
    >
      <div class="slds-truncate" title="{period.Forecast_Period_Start__c}">
        <lightning-input
          type="date"
          name="input2"
          label="Period start date"
          value="{period.Forecast_Period_Start__c}"
          onchange="{handleChange}"
        >
        </lightning-input>
      </div>
    </td>
    <td
      data-label="{period}"
      key="{period.index}"
      data-id="{period.Id}"
      data-field="Forecasted_Days__c"
    >
      <div class="slds-truncate" title="{period.Forecasted_Days__c}">
        <lightning-input
          type="number"
          name="Forecasted_Days__c"
          label="Number of days"
          value="{period.Forecasted_Days__c}"
          onchange="{handleChange}"
        >
        </lightning-input>
      </div>
    </td>
  </tr>
</template>

lwc.js

handleChange(event){
  const currentRecordId = event.target.closest('td').dataset.id;
  const fieldName = event.target.closest('td').dataset.field;
  this.forecastRevenues = 
    this.forecastRevenues.map(r=>{
      if(r.Id===currentRecordId){
        r[fieldName] = event.detail.value;
      }
      return r;
    });
}