You didn’t say if you wanted to use CEM-style or GL-style Practice Papers or what subjects you are looking for.
100 credits would ‘buy’ you, for example, one 2018 mock test paper or 8 VR practice Papers + 5 maths practice papers.
1000 credits would enable you to buy many of the tests I have noted below. There are so many suitable practice papers (some in the ‘archived’ tests section)that you would need to browse them online to decide what you needed first.
Just to give you some idea – There are 10 CEM practice papers = 25 credits each. Many people also use the many past mock test papers as practice papers -which are a hybrid tests of CEM + GL + CSSE etc. style tests e.g. 2018 mock test = 100 credits per test (Don’t forget that you are given 100 free credits for each mock test booking this year and so most people use those to try out the 2018 mock tests.)
There are in addition to the above:
8 VR and 5 maths Practice Papers (GL-style) = between 5 and 8 credits each
Lots more previous mock test papers = ideal practice tests (GL-style) 2009, 2010, 2013 – each examined subject = 20 credits each – less than last year’s mock tests.
And much more …
I hope that helps but it is best to have a look in COAsT at the practice papers and past mock test papers.