How Many Experiments to Run?
The size of the design space and the number of experiments are closely connected.
To make the most out of Design of Experiments and Bayesian Optimization, it is important to choose a reasonablysized reaction space and according number of experiments.
Exploring the Reaction Space  Design of Experiments
Minimum number of experiments. The minimum number of experiments can be estimated by adding the number of possible values for each parameter. For example, if you want to test 4 ligands, 3 solvents, 2 bases and 2 concentrations, you should be aiming for at least \(4+3+2+2=11\) experiments.
An exception is when using descriptors for a parameter with a large number of possible values. For example, it is possible to screen 30 ligands using 10 or 20 experiments, if the descriptors reflect their reactivity well.
Using multiples of number of parameters. It is good practice for number of experiments to be a multiple of the possible values of number of parameters. This ensures that each parameter is tested the same number of times.
Using the same example of 4 ligands, 3 solvents, 2 bases and 2 concentrations, a good number of experiments could be 12, or better 24.
Rule of thumb. The table below summarizes common number of experiments for different numbers of parameters and values.
Number of Possible Values for Each Parameter 
Full Factorial 
Recommended Experiments 

\(2 + 2 + 2 \,(+ 2)\) 
8 or 16 
4, 8 
\(3 + 3 + 3 \,(+ 3)\) 
27 or 81 
9 
\(4 + 4 + 4 \,(+ 4)\) 
64 or 256 
16, 32 
\(4 + 2 + 2 + 2 \,(+ 2)\) 
32 or 64 

\(4 + 4 + 2 + 2 \,(+ 2)\) 
64 or 128 

\(3 + 3 + 2 + 2 \,(+ 2)\) 
36 or 72 
18 
\(4 + 3 + 2 + 2 \,(+ 2)\) 
48 or 96 
12, 24 
For example, if you want to test 4 ligands, 3 solvents, 2 bases and 2 concentrations, there are \(4+3+2+2\) possible values for each parameter (last row of the table), and the recommended number of experiments is 12 or 24.
Finding the Best Conditions  Bayesian Optimization
Unlike for DoE, the number of experiments for Bayesian Optimization is not as important, so you can decide based on your experimental setup. A good starting point is 5  20 experiments per batch, although doing more experiments or just a single one at a time should also work.