Answer: 20.
Let $n$ be the number of co-funders and $d_i$ be the value of the $i$-th director, $i = 1, \ldots, n$. By condition,
$$d_i = 3 \cdot \frac{d_{i+1} + d_{i+2} + \dots + d_n}{n-i}.$$
So,
$$\begin{align*} d_{i-1} &= 3 \cdot \frac{d_i + d_{i+1} + \dots + d_n}{n-i+1} = \\ &= 3 \cdot \frac{3 \cdot \frac{d_{i+1} + d_{i+2} + \dots + d_n}{n-i} + d_{i+1} + \dots + d_n}{n-i+1} = \\ &= 3 \cdot \frac{(n-i+3)(d_{i+1} + d_{i+2} + \dots + d_n)}{(n-i)(n-i+1)}. \end{align*}$$
Therefore, $\frac{d_{i-1}}{d_i} = \frac{n-i+3}{n-i+1}$, $i = 2, \ldots, n$.
Multiplying these equalities, we obtain
$$\frac{d_1}{d_n} = \frac{d_1}{d_2} \cdot \frac{d_2}{d_3} \cdot \dots \cdot \frac{d_{n-1}}{d_n} = \frac{n+1}{n-1} \cdot \frac{n}{n-2} \cdot \dots \cdot \frac{4}{2} \cdot \frac{3}{1} = \frac{(n+1)n}{2}.$$
By condition, $d_1/d_n = 210$, so $(n+1)n = 420$ which gives $n = 20$.