How much cellulose is present in cotton? 

>90%

More than 90% of a cotton fiber consists of cellulose.  The exact percentage of cellulose in a specific cotton fiber sample depends on whether the fiber is wet or dry, and on how much effort has been made to extract the non-cellulosic components.  For example, when it is equilibrated at 65% relative humidity at 20^oC, cotton contains slightly more than 7% of absorbed water by weight.  Hence the percentage of cellulose in a cotton fiber can range anywhere from just a little over 90% (wet, and with none of its non-cellulosic components extracted) to >99% (dry, with most of its non-cellulosic components extracted).