Are you pigs Male or female? If they are female, then adding more is fine, in theory, to do. Many females can live together in a herd quite happily. Remembering that any new pigs might not get on with your pigs etc and they simply not want to bond. There’s always a risk when you choose new pigs, rather than the pigs choosing their own new friend, that they just won’t like each other. When adding new pigs to your existing pigs, you must always be prepared for any new bondings to fail and must be prepared to have two separate cages so that they can live in permanently separate pairs if they don’t like each other.
If they are Male, then you cannot add any more pigs into the cage with them. Only two males can live together, any more than that and they will fight.
Letting pigs live next door to each other before doing the formal bonding can be helpful as it gives them all a chance to get to see who the neighbours are before being put in the same enclosure.
there are many guides in the bonding section which can help you so do take a look