The sutta refers to 7 kinds of offerings to the Sangha (bhikkhus/bhikkhunis or monks/nuns) (satta saṅghagatā dakkhiṇā)
(1) One gives to both sanghas (of monks and of nuns) headed by the Buddha—this is the first offering to the order.
(2) One gives to both sanghas (of monks and of nuns)27 after the Buddha has passed away—this
is the second offering to the order.
(3) One gives to the sangha of monks—this is the third offering to the order.
(4) One gives to the sangha of nuns—this is the fourth offering to the order.
(5) One gives, saying, “allocate this for monks and nuns from the sangha” [256] —this is the fifth offering to the order.
(6) One gives, saying, “allocate this for monks from the sangha”—this is the sixth offering to the order.
(7) One gives, saying, “allocate this for nuns from the sangha”—this is the seventh offering to the order.
You asked: “there is a statement that even 1 pair of robe/cloth can be offered as Sangha Dana / Saṅghagatā Dakkhiṇā. in that case, is offering by Gotami to Sangha considered as type 1 Saṅghagatā Dakkhiṇā (from 7)?”
– Yes. Since it was a general offer without reference to a specific bhikkhu or bhikkhuni.
– The Sangha will decide who will get the robe.
Per your next question, it does not matter how much one gives. One gives based on one’s capability and/or the needs of the Sangha. If one can afford only one robe, one should make the offering to the Sangha. They will decide who will get the robe.
“I heard that Sangha Dana can be considered as Sangha Dana if there are at least 4 Bhikkhus or more at that place when an offering is made?”
– I don’t think that is true.
I think you should be able to get the answers to the other questions the same way. It does not matter how much one gives. One gives based on one’s capability.
– What matters is the right intention.