Yes, Scots speakers were recorded at around 1.5 million in the last census. The number is probably much higher, however due to the complicated relationship with the language that many people have for various reasons (discouraged at home or school etc) many daily Scots speakers still deny its existence as slang or dialect.
The comparison could be made to Danish and Norwegian, when you know one the other is much easier to pick up. The most difficult part would be finding speakers to actually practice with i would assume.
Yes and no, they would most likely understand parts or enough to go off of context. This of course depends on if you are speaking only Broad Scots or whatever varying shade of Scottish Standard English the situation would demand.
There is great resources for the Scots leid from language courses and books to the ever helpful Dictionary of the Scots Language. Books such as this and this can be found for free on sites like library genesis. The DSL is probably rather much for someone just starting off, so going off a response to one of my own responses to a post a bit like yours a while ago they recommended https://scots-online.org/ instead to begin with.

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Reddit mentions of Modren Scots Grammar: Wirkin wi Wirds