Is there any point in buying over 100 of something except Cursors, Grandmas and Time Machines?

It depends on your motivations for playing but the newest target is 200, and beyond that, there are no imperative reasons to purchase any more. If your only goal is to have everything currently available, then you can stop once you have all achievements, all upgrades, 240 cursors and 200 of everything else, got all the seasonal purchases and clicks, 7777 lucky cookie clicks, and have reset 12.5 quintillion cookies away as there is currently nothing beyond that.

You may optionally want to try to optimize CPS gains by balancing out cost/cps for building purchases in order to make more efficient progress later, but that is entirely optional and may not prove to be worth your effort depending upon what is to be added. Because the cost/cps is better on some of the lesser buildings when compared in equivalent quantities of other buildings, your most efficient CPS purchases will be from some of these cheaper buildings. Essentially you would calculate buildingCost/(cps/building)=cpsCost for your most profitable building and then purchase enough of every other building to get the cpsCost for each building to be approximately the same value as was calculated for the most profitable (because they will have the worst cost/value if you don't try to balance the cpsCost). It should be noted that the cost/value relationships change as you purchase upgrades. Having a spreadsheet helps.

You need 128 Grandmas and 128 Farms for the Mathematician achievement.

The accepted answer is not current nor accurate.

Because Cursors produce more CpS based on the number of other buildings you have, if you have enough Cursors then it does make sense to purchase more than 200 Farms / Factories / Mines / Shipments. And, because it's profitable to get 280 Cursors (around 10 quintillion cookies) long before getting Kitten Managers make sense, you will also around that time will have enough marginal gain from more cheap buildings on Cursor income alone.

A good rule of thumb would be to keep your cheap buildings costing around 5% of what your Grandmas cost. For example, if you have 284 grandma's (16.792 quintillion for the next one), you should have about 245 farms (360.451 quadrillion for the next one). Note that those prices assume the Christmas Upgrades and the Faberge egg.