The normal pH range of saliva is 6.2-7.6 (PubMed, 2013). The enamel starts to dissolve when the pH of the saliva or the fluid beneath the plaque falls under about 5.5, which is known as "critical pH" (Journal of Canadian Dental Association, 2003).
The pH of saliva falls, for example, when you drink acidic beverages, such as cola with pH 2.5-3.4 (PubMed, 2010).
The pH of the fluid beneath a dental plaque falls when the bacteria normally present in mouth convert certain nutrients, mainly sugars sucrose, glucose and fructose into acids (jisppd.com). Major sources of these sugars are table sugar, sweets, soft drinks, fruit juices and honey.
In one small study, oral rinse with sodium bicarbonate (baking soda) increased the pH of saliva, which could, theoretically, help to prevent tooth decay (PubMed, 2017).
I haven't found any human trials in which toothpaste would be used to increase salivary pH.
Salivary characteristics and dental caries: Evidence from general dental practices (PubMed, 2013)
Our study findings show that salivary characteristics were associated
weakly with recent dental caries experience, but we did not find
consistent trends among the three age groups. Thus, one should
interpret with caution an assessment of salivary consistency, salivary
pH or salivary flow rate to determine the caries risk of all patients.
In conclusion, one may not need to focus on salivary pH, but try to avoid known risk factors for caries, mainly sugary foods and bad teeth hygiene (Mayo Clinic).