T = your logical trials vector
n = 20; % number of trials to keep
f = find(T); % find the location of the 1's
f = f(randperm(numel(f))); % randomize the find results
T(f(n+1:end)) = false; % get rid of the other trials beyond the random 20
Or if you need to do this repeatedly for the same T, do the f = find(T) only once, then repeatedly do this
R = T;
f = f(randperm(numel(f))); % randomize the find results
R(f(n+1:end)) = false; % get rid of the other trials beyond the random 20
Alternatively, one could start with an all 0's matrix and then repeatedly randomly fill with 1's. E.g.,
R = false(size(T));
f = f(randperm(numel(f))); % randomize the find results
R(f(1:n)) = true; % randomly fill in the 1's
