I am using multiple ISR881s in hub and spoke scenario, all configured as "eigrp stub connected". I am wondering how many EIGRP routes can be received from two HUB neighbours and safely maintained by 881 (256MB of DRAM).
I do not believe that there is any particular number that would be the limit of how many EIGRP routes. The limit would depend on what else you have configured that is using memory on the router in addition to the EIGRP topology table and the IP routing table.
If you are doing hub and spoke with EIGRP stub connected at the spoke then a good practice is to limit the routes that are advertised from the hub to the spoke. This can easily be done with eigrp summary address commands on the hub or could also be done by implementing distribute lists in EIGRP on the hub. I have done this (both ways) and it works quite well.