There's probably a better way to do this but I think this will generate what you want:

nsplit = function(X,n){
 p = X/sum(X)
 diff(round(n*cumsum(c(0,p))))
}

where X is a vector of sizes and n in the total. Example:

> nsplit(c(1,2,3,4,5),123)
[1]  8 17 24 33 41

Those numbers sum to 123 and are approximately in the ratio 1:2:3:4:5. See also this:

> nsplit(c(1,2,3,4,5),10*sum(1:5))
[1] 10 20 30 40 50

Note if any of the ratios are small or n is small then you will get zeroes:

> nsplit(c(1,2,3,4,50),20)
[1]  0  1  1  1 17

There's probably a better way to do this but I think this will generate what you want:

nsplit = function(X,n){
 p = X/sum(X)
 diff(round(n*cumsum(c(0,p))))
}

where X is a vector of sizes and n in the total. Example:

> nsplit(c(1,2,3,4,5),123)
[1]  8 17 24 33 41

Those numbers sum to 123 and are approximately in the ratio 1:2:3:4:5. See also this:

> nsplit(c(1,2,3,4,5),10*sum(1:5))
[1] 10 20 30 40 50

Note if any of the ratios are small or n is small then you will get zeroes:

> nsplit(c(1,2,3,4,50),20)
[1]  0  1  1  1 17

I think you can then feed those numbers into spsample to generate points, but you'll have to do it one polygon feature at a time since spsample will only handle a single n argument.