The garden harbors a number of ant species of which these are the largest and most conspicious. They are Jet ants (Lasius fuliginosus) [1,2].
I do not know how many colonies of this species are actvie but I suspect it is only one because all ant more or less converge or diverge into or from the same direction. Probably they arenesting inside one of the outer walls which they enter through holes int he ground.
Every year there are hundreds of ants active in a more shaded part of the garden. Here the growth is more dense and continuous with ivy, dogwood and common nettle. The plants contain large quantities of aphids, which are intensively kept as cattle by the ants.
Some parts of the plants are covered with aphids and ants. The branch tips often contain many ants and brushing them will result in ant on ones arm. In case this happens they often react agressive and will bite bare arms and hands, which is not super painfull but it is an annoying pain.
References1 Nederlands Soortenregister
2 Boer, P. 2015. Mieren van de Benelux. Jeugdbondsuitgeverij, 's Graveland.