When adding members to a marketing list, among other things pay attention to the number of members added. By default, there is a limit set at 50,000.

If working with an on prem. deployment, this can be adjusted in the MSCRM_CONFIG database, in the DeploymentProperties table, under AggregateQueryRecordLimit. This value also controls dashboard chart queries, so choose carefully how you modify it.

More details on MSDN at gg334634.

Obviously you can not modify this for CRM Online.

Please note, this limit does not apply to Dynamic Marketing Lists.