Typically you want to have atleast 2 pages empty, especially if you are travelling into the country from abroad.
The reason for this is that your visa sticker/stamp is usually attached to one page, and a fresh page is used to indicate every entry you have into the country in the form of an Entry stamp at Immigration Control.
I would not worry about it too much if you are running out of space - a visa on a previous passport is just as valid as a visa on a current passport (assuming it is still valid time and conditions wise, etc.).
Though if you are planning to apply for a new passport soon it may be a better idea to do so first as carrying two passports joined together is slight more troublesome than carrying just one.
