Typically the deposit will be equal to the first months rent. For furnished rentals, it is becoming more common for deposits to be first and last months rent.
The reason for this is because some people use their deposit to pay their last months rent. Which means that if the deposit was only one months rent, the landlord could be in trouble if there is any damage to the unit. Certain types of damage can take days or even weeks to show as well.
A good rule of thumb is, whatever the monthly rate is, multiply that by 3 and that is what you’ll have to pay before you move in.
Keep in mind, everything is always negotiable and this is generally a great item to negotiate.
It all comes down to trust.
Keep in mind, the entire purpose of a deposit is to mitigate risk. If you can speak with the landlord, preferably in person, and convince them that you are trustworthy, your chance of negotiating the deposit increases dramatically.