The electoral legal framework will generally specify the number of members of the EMB. It is not possible to specify an exact number for multiparty EMBs whose membership is determined by a criterion such as party representation in the legislature, as this number may vary over time.

The number of members of an EMB varies considerably worldwide, and need not be related to the size of the country. As of 2006, in the small country of Lesotho, the EMB has three members, whilst Nigeria, a much bigger country, has a 13-member EMB. However, countries like Canada, despite its sizeable land mass, and India, with about 600 million voters, have one-member and three-member EMBs, respectively. Having a larger number of members of the EMB may provide for broader representation, whereas a smaller number can facilitate discussion and decision-making.

EMBs which include political party nominees tend to have higher numbers of members, so as to accommodate a credible range of political interests, while expert-based EMBs tend to have fewer members. Having an uneven number of members of the EMB ensures that simple majority vote decisions can be made without having to resort to measures such as giving the EMB chair a casting vote.

In some cases, electoral law allows for the appointment of alternate or deputy members to EMBs. Under certain circumstances, a deputy member can then automatically take the place and exercise the powers of a member.