The number of pollen grains per anther is generally genetically determined per species. However, some research suggests that applying fertilizers can increase this limit somewhat (e.g. NPK), but that this varies between species to what is effective and what increase is possible. For the overall pollen output applying fertilizers have been able to increase the absolute number of flowers or male reproductive organs in some species, increasing pollen production by proxy. See below for a few references:

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There are several ways to increase the number of pollen grains in the anthers of plants:

Genetic improvement: By breeding or genetically engineering plants with higher pollen production, the number of pollen grains in the anthers can be increased.

Nutrition: Providing plants with adequate levels of essential nutrients such as nitrogen, potassium, and phosphorus can increase pollen production.

Temperature and humidity: Pollen production is optimal at specific temperature and humidity ranges, so controlling these environmental factors can increase pollen production.

Light: Adequate light is essential for pollen production, so providing plants with sufficient light can increase pollen production.

Pollen transfer: Pollen transfer can be increased by utilizing manual or mechanical pollen transfer methods to increase the number of pollen grains that reach the pistil.

Pest management: Pests and diseases can damage the anthers, causing the number of pollen grains to decrease, so proper pest management is important.

Hormones: Plant growth regulators such as cytokinins, gibberellins, and abscisic acid can be applied to enhance anther development and pollen production.

It's important to note that some plants have more pollen than others, and the amount of pollen can also vary depending on the plant's age and the growing conditions. Therefore, it's important to conduct a thorough analysis of the specific plant species and growing conditions before implementing any methods to increase the number of pollen grains in the anthers.