I asked a couple of people to weigh in on your question and here are their responses:

1. One question needs to be answered first: “What is the function of the
‘trustees’ (an office not found in Scripture). If by trustee one means “elder”
(in biblical terms) than they are to shepherd the flock (e.g. Acts 20:28). The
reasonable question, then, is “How many people can we expect an elder to
shepherd?” (This will probably differ a bit from one individual to another.)
That will set you up to do the math.

If, however,
“trustee” really means “deacon” (the pastor is the elder, the elders are the
deacons, and the trustees are the deacons – I suspect this is what is meant)
then I think we find our guidance in Acts 6. You appoint the number of deacons
necessary to carry out the tasks you are doing. Thus, you identify the
ministries that need leadership, determine how many are needed in each, and add
up to find the total number needed. That’s as close to the Scriptures as I can
get.

2. You might look at the Willow Creek Association. The have a number of resources that might be of assistance.