] 1- Is the bathrooms in JA's time looks like the one in P&P2?

Up until about the 20th century, even the rich really didn't have rooms devoted only to bathing. Wherever the bathtub was set up was the "bath"room - usually the person's bedroom, as we see with Darcy.

] 2- Is it necessary for a gentleman to keep a servant in the bathroom helping him?

It was very common for those who could afford it to have a servant help them bathe - in fact, I understand that aristocratic ladies were often washed by their maids as if they were children! (I don't know about the gentry.)

] How could they handle their baths in these days especially that they don't have pipes for clean water and pipes for sewage?

If you read accounts of those days from the servants' point of view, every time an employer wanted warm water just to wash their face or hands, somebody had to heat the water down in the kitchen, then bring it upstairs in a kettle or pitcher, making as many trips as necessary (and those kettles of water were heavy, too). When somebody wanted to take a bath, that meant a lot of trips up- and downstairs for some servant!