Marketing Efforts share ID's with Appeal Mailings, Marketing Acknowledgments, Membership Efforts, Sponsorship Efforts, and Public Media Efforts.
So, if a Marketing Effort is created and then an Appeal Mailing, each parameter would be assigned a sequential ID. For example, #150 ( Marketing Effort ) and #151 ( Appeal Mailing ). Then, if another Marketing Effort is created, it will be assigned ID #152.

Also, if a Marketing Effort or Appeal Mailing is deleted, the ID will not be reused and CRM will assign the next available value. Because of these reasons, it is possible to see skips in Marketing Effort ID's. 

To see all of them, create a marketing effort query and using no critieria, output the effort ID and the type. This will show the different types of marketing efforts (i.e. Direct Marketing, Sponsorship) along with the number for each one.