Batch codes were first proposed by Ishai et al., an (n,N, k,m)-combinatorial batch code was defined by Peterson et al. as a purely combinatorial version of batch codes. It is a system consisting of m subsets of an n-element set such that any k elements can be retrieved by reading at most one (or in general, t) elements from each subset and the number of total elements in m subsets is N. For given parameters n, k,m, the minimum N, denoted by N(n, k,m), is of particular interest, which has both theoretical interests and strong practical motivation. So far, for k ≥ 5, m+3 ≤ n <, precise values of N(n, k,m) have not been established except for some special settings of the parameters. In this article, we obtain N(m+3, 5,m) = m+11 (m ≥ 7), N(9, 5, 6) = 18, N(m+3, 6,m) = m+13 (m ≥ 8), N(10, 6, 7) = 21. Our results partly answer an open problem of Paterson et al.