To the best of my knowledge, I didn't see anywhere in the Cisco document specifying about the number of users supported in 1811 router. Tell me the link or the document you are come across for this specification.
I have raised a TAC case on this and had it confirmed that this Cisco 1801ISR series has a mac limit of 50, so only 50 users in total can use this box from the wireless or LAN connectivity.
I must admit the spec is very vague and some of the installation pdf show connection of a hub to the switch port. It also states in the data sheet that user support is 50, however I thought that meant 50 leap/wireless users.
I think it is disgracefull for cisco to have this design critical information and bury it deep in some data sheet.
It appears at one time Cisco may have placed hardware limits on the # of users in the 18xx/87x/85x routers, but that is no longer the case. The user limits are merely recommendations, and are not enforced by any means. I just confirmed this via a TAC case.