Auto adding a number in a different cell after an action is taken

Hi. I am making a spreadsheet that lists the items we service at work and shows the next service date. I have also made a word document that is to be used as a printable work order that fills with information from the spreadsheet via mail merge. On the spreadsheet the date the work order is issued will be entered manually when the job is raised. The work orders need a number assigned to them for each job, these will range from 1001 onwards. Is there a way that when the work order issue date is entered it will auto update the work order assignment number by adding 1 on? There are 3000 items and the jobs will not be done in order. I haven't been able to find a way to do it but I am a mere novice, if anybody can help it will be greatly appreciated.