If you only need 250 hosts as a max, then a /24 (regular class C address) would do, and you can just ask ARIN for how ever many class C network addresses as you need. You have a requirement for 10 networks, so you can just buy 10 Class C addresses. Future expansion then simply requires buying more class C numbers.
If you need future expansion without the extra potential cost of buying more addresses, then a /23 class B address will give you up to 510 hosts (more than double your original requirement) and 128 networks, which is plenty more than 10, so you have room for expansion of both hosts and networks, as well as the freedom to VLSM and still retain useful network sizes.
If cost is an issue, you may find that a /23 address range is a lot cheaper than a /20 range from ARIN or your ISP. If the cost is the same then buy as large an address range as is available, but IME, it usually does come down to money. However if the assignment is to get as close to 250 hosts as possible regardless of any other considerations, then a single class C address per network address range as required will do.
