Traditionally, the guest list would be decided by the bride's parents, in consultation with the groom's parents; nowadays, it is more common for the couple to devise their guest list, in consultation with their parents.
In any event, the bride's parents (or whoever is acting as hosts) should make the final decision as to numbers to be invited - at least as far as the reception is concerned, as they should have an idea of how many guests they can afford to cater for.
It is reasonable to assume that both the bride and the bridegroom - and their parents - will wish to invite similar numbers from amongst their own relatives and friends, and giving each group a third of the guest list to choose is often the fairest way of dealing with things.
When compiling the guest list don't forget to add the name of the celebrant and their partner, as it is always courteous to ask them along. More importantly - don't forget to add yourself to the list!
If you have either limited space or budget, you can always invite any extra people to the evening reception only. Although in years past people may have taken offence at not being invited to the ceremony, invites to the evening reception only are becoming more and more popular, and most people are happy to join in the celebrations at any point of the day.