Hello Ramanjot Kaur , usually when using 5ml cultures gives us concentration around 250ng/ul (usually high yielding plasmids) after incubating overnight at 225RPM (This value is constant in most cases. it is highly advisable to use 250ml cultures and do a maxi preparation. Apart from that, you can also do the ethanol wash to increase the plasmid yield and add lesser volume of buffer to dissolve the plasmid in case if you are using the 5ml culture.

Kousalya Lavudi Thank you so much, when we have to give ethanol washing, i elute in 20 ul only

You can often release more of the bound plasmid by either passing the elution buffer through the column a second time or by using hot (80c) buffer ( or water) to elute the bound dna

Plasmid yield is varying on what type of plasmid( Refer the copy number of that plasmid) you are using and in which strain you grown.

If you are using a expression vector like pet vector take 10 ml overnight culture and use QUIAGEN miniprep plasmid isolation kit as it's spin column capacity is high, you can easily get higher concentrations of your desired plasmid. Kindly don't shake too much before the phase seperation. One another thing don't spin the overnight culture over 4000rpm.