Take a look at the CCM SRND below. The maximum number of devices for a 7835 is 2500. The argument could be made that not all devices will be utilizing the TFTP/MOH server at any given time, however it is possible that all phones would be booting up at the same time in certain scenarios. Cisco also recommends that you use the same server model throughout the cluster. So, if you're using 7835s, I would recommend 2 7835s for TFTP/MOH. This offers scalability and redundancy for these services.
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_implementation_design_guide_chapter09186a00806e8c0a.html#wp1085323
Hope this helps. If so, please rate the post.
Brandon