I am presuming that these are elections where nominees have to submit their applications to the Borough / District Council? If that is the case, those interested will have to be eligible to apply and they would be aware that there would be a formal ballot if more applicants apply than positions on the council... They will also have to get seconders and I believe you can only second one other person, especially if you are standing for election youself.
I think it is the responsibility of the existing councillors to encourage new membership but there is a risk that if the number falls below the positions on the council, they are uncontested and this could create an imbalance but that is the nature of the beast.
If there is a genuine concern, then the best route would be for all the Councillors to encourage their friends, get more applicants than needed and then pay for an election but that will cost you....
Consider the co-option method after all.... we have a formal policy but some parish councils are so desparate for new councillors that the bloke down the pub or your next door neighbour is press ganged...