It's simple. There are two things that define how many guests you can have at a wedding: cost and location.
However, the combination of cost and space with the thorny problem of parental input can cause tensions. It’s time for evaluation, compromise and realism.
Then ask these questions: how big is the venue and what can the budget afford? It's unavoidable that guests will have to be cut. Separate the guests who have to be there - the A list - from anyone who is not essential - the B list.
You can invite around 10% more guests than the target number, because this is the average percentage of those who won’t be able to come.
Traditionally, the bride's parents pay for the Big Day, which can give them the upper hand when extending invitations. Even if you pay for it all yourself, expect some parental input on the guest list. Be respectful. Try and honor at least some of their wishes. A solution is to give each set of parents a fixed number of people they are able to invite.
When it comes to work colleagues, a useful rule is to invite those you are friends with outside work.
If your edited list still contains 300 names and your location only holds 175 for example, you will have to set some cutting parameters that won't make you feel bad.
Which acquaintances are so important they have to be at the wedding. The focus needs to be on those people who are most relevant to your life in the present, and who are likely to still be relevant in five years’ time. Couples who you are not close to anymore do not have to be invited simply because you went to their wedding ceremonies.
At the end of the day, it's good to make a head-count database for use throughout your planning process. Make columns for contact info as well as RSVPs and gifts, plus any other relevant information. If you want to keep costs low, it's time to be brutal and reduce your guest list.