hi zoe, sorry for your losses. For NHS testing it's three in a row (i.e. with no babies in-between), except in cases of losses after the first trimester. One of the reasons given for this is that statistically it isn't uncommon to have two miscarriages (though that doesn't help when it happens to you!), so lots of women who experience this don't need the tests.
Many doctors will accept referrals for testing after two losses for private patients.