Not automatically. The number of days and the number of entries are two different parts of the visa.

A Dubai 30 day visa may be available as either single entry or multiple entry, depending on your passport and the option offered to you. A single-entry visa is for one UAE arrival. Once you leave the UAE, that entry has been used.

If your itinerary looks like:

Dubai → Muscat → Dubai

then you should check the multiple-entry option before applying.

But if your trip is simply:

India → Dubai → India

then a single-entry visa may be enough.

This is why it is better to finalise the broad itinerary before paying for the visa. Dubaivisa.com specifically advises travellers to choose multiple entry only when their trip actually requires leaving and returning to the UAE.