Hi,
I don't think thats the case. Cisco 770 Router is only used for a small Lan design and if I am not wrong cisco says that it should handle only 8 Pc's(you can crosscheck in the Cisco 770 manual) but still you are able to use 20 Pc's.
I would suggest you to go for one more 770 router and with one more ISDN line as it will improve the performance of the Lan and all the users will not find the speed of the internet slow while browsing it. Also you will can accomodate 5 users for the future growth as total are 35 and you can go for 40 users on 2 seperate routers.
As per my own experience on Cisco 775 Routers thats my advice and I hope it will help you.
Regards,
Sameer