Please let me know how many are coming, if any.

请告诉我，如果有人要来的话，有多少？

As you only have a short stay in Shanghai, you probably will not be able to do much more sightseeing. But if I can help in any other way, please let me know.

由于您在上海只能做短暂停留，您可能无法做太多的观光。但如果我能在任何其它方面帮上您的忙，敬请告知我。

If you do please let me know.

如果您这样做了请告之我。

Please let me know how it goes.

让我知道你进行得如何。

If you disagree, please let me know.

如果你不同意的话，请让我知道。