One transfer per use year (it is determined by use year not calendar year). If multiple contracts in same use year, still only one total transfer, either in or out. If you have two different use years (e.g., Feb and June) and thus two different membership numbers, you get one transfer for each of those use years. Moreover, with any one use year membership, you can actually make two different transfers at the same time, one for the current use year and one for the next use year, e.g., if you have an April use year, you could today transfer April 2021 use year points and April 2022 use year points.
Only exception to one transfer per use year rule is for those who have two or more use years/memberships (e.g., you have both a Feb use year account and a separate June use year account) and want to transfer points between their own use year membership accounts. MS usually allows that to be done more than once per use year.