I'm not sure if this is appropriate for your case, but usually map is a good answer.

If this is your code using a for loop:

<tbody>
    for (var i=0; i < objects.length; i++) {
        <ObjectRow obj={objects[i]} key={i}>
    }
</tbody>

You can use map to write 一> like this:

<tbody>
    {objects.map(function(object, i){
        return <ObjectRow obj={object} key={i} />;
    })}
</tbody>

ES6 syntax:

<tbody>
    {objects.map((object, i) => <ObjectRow obj={object} key={i} />)}
</tbody>

Think of it as if you were just calling a JavaScript function. You cannot use a for loop to pass arguments to a function call:

return tbody(
    for (let i = 0; i < numrows; i++) {
        ObjectRow()
    } 
)

See how the function tbody is passed as a parameter to the for loop - causing a syntax error.

But you can create an array and pass it as a parameter:

const rows = [];
for (let i = 0; i < numrows; i++) {
    rows.push(ObjectRow());
}
return tbody(rows);

You basically use the same structure when using JSX:

const rows = [];
for (let i = 0; i < numrows; i++) {
    // note: we are adding a key prop here to allow react to uniquely identify each
    // element in this array. see: https://reactjs.org/docs/lists-and-keys.html
    rows.push(<ObjectRow key={i} />);
}
return <tbody>{rows}</tbody>;

By the way, my JavaScript example converts almost exactly the same thing as the JSX example. Try using Babel REPL to get a feel for how JSX works.