How to add multiple taxonomy term to a single node in drupal 7?
Go to Term Reference Field settings of the content type and change 'Number of values' to  your required value.

For example if you want to allow 2 terms per node change the allowed number of values to '2' from select list. If you want to allow more than 10 or so change the option to 'Unlimited' so that you can insert multiple taxonomy terms to a node separated by 'comma' (,).

Here I've provided screenshot to allow unlimited values. Here 'SECONDTERM' is my    Term Reference of Autocomplete term widget field label name.